The shop has been open for about five minutes when the first customer of the day triggers a landslide of books in the northeast corner. James Peacock is a diligent collector, and a highly experienced climber of the teetering stacks that rise from this shop floor. This morning, however, he is unlucky – extracting one slim volume from the base of a tower of hardbacks, he sets off a terrible wobble.

“I bloody knew that was going to go,” whispers Peacock, too late, with the sound of the cascade still echoing through acres of wood pulp.

“Happens all the time,” says Eddie McGonigle from behind the counter, which has long since been reduced to another heap of stock.

Established in 1972 by Eddie’s brothers Joe and Gerry, Voltaire And Rousseau is now the oldest second-hand bookshop in Glasgow, and surely the untidiest. “Some people hate the messiness,” he says. “But we get a lot of others who absolutely love the idea of finding a book at the bottom of a pile.”

James Peacock and his father – who is also named James and is also a collector – must belong in the second category.

They drove over from Edinburgh first thing, and were knocking on the shop’s front door before Eddie had unlocked it. These Peacock boys tell me they do constant rounds of the country’s second-hand bookshops, and confirm that there is no other quite like this one. “Organised chaos,” declares James Senior. “That’s the fun of it.”

He pulls up a history of scientific mining from a loose mound of non-fiction, and a few titles relating to the ancient burgh of Stirling, while James Junior adds to his own armfuls of purchases with a study of Scottish folklore and a guide to translating Sanskrit.

“This might be the last place left in the UK where you still want to take your time and really do a proper dig,” he says. One blogger recently went further, in an online travel guide to obscure and rewarding detours (www.nothingtoseehere.com), describing Voltaire And Rousseau as “the last real bookshop on the planet”.

If Voltaire himself were still alive, the insufferable ironist would undoubtedly LOL to read such a claim made on the internet, for a store that has literally no room for a computer, and apparently no need for a website. (Poor Rousseau, so wary of modernity, would probably choke to death on our text-speak for “laugh out loud”.) “We have nephews who are proposing to do us one ...” says Eddie, without sounding particularly fussed . The McGonigles are not too old-fashioned to auction the odd rarity on eBay from home, or run elusive items through a search engine. But their business model – if that term applies here – remains strictly analogue.

“We’re probably selling less and less these days, and I think it’s because so many people are buying their second-hand books from Amazon or Abe. The thing about that is, you need to know exactly what you’re looking for, the author or the title at least. “Let’s say you’re interested in Byzantine art. If you came to us for a look around instead, you might find a book on the subject that you’d never even heard of. It’s a social thing as well. Rather than using a machine, you are dealing with people, and getting to know them.”

In the course of a full Monday spent within these two cluttered rooms on Otago Lane, I will meet buyers, sellers, and searchers, such as Danish translator Nina Enemark, who is desperately seeking Madeleine: One Of Love’s Jansenists. “Aren’t we all,” I joke, but it’s actually a 1919 novel by Hope Mirlees, so long out of print even Eddie hasn’t heard of it.

Cora McCormack and Alana Ross, both drama students at Stowe College, are shopping for a vintage copy of Alice In Wonderland as a birthday gift for their friend, Beth. “She likes old things,” explains McCormack. “Oh aye, those are lovely old books up there,” says Ross, gazing like a born stage actress to the shelves at the top of a rickety ladder. “I might not read them, but I’d buy them just to have them in my house. How pretentious am I?”

Musician William Graham is offloading three boxes of his brother’s discarded literature. “Could ye no just give me £20 for the lot?” he asks. He gets £15, and considers this fair.

John Cromar, an exam invigilator, gets the same for the “popular science and second world war stuff” he is selling on behalf of a departing flatmate. He often buys his own books here. “Why pay £14 for a new copy if you can get it for £3? The only problem is, it might be hidden right down underneath a lot of other stuff. I’m a bit too inhibited to go raking through everything.”

I know what he means, having already witnessed countless little slips and several structural collapses among the rows and shelves and load-bearing walls of paperbacks. I will later cause some wreckage myself, and interpret this as biblical punishment for my eagerness to peruse a well-thumbed but deeply buried copy of De Sade’s 120 Days ­­ Of Sodom.

Like many lifelong bookworms, I read too much into everything.

After hours of random browsing – Goethe’s Faust, The Teachings Of The Compassionate Buddha, Harry E Wedeck’s Dictionary Of Aphrodisiacs – the shop begins to seem like a serviceable metaphor for the universe.

“It would be nice if it were more ordered,” says regular customer Chris Shaw, who has just bought one of Terry Pratchett’s Discworld novels, but not the one he was after.

“It’s a bit annoying sometimes, because the thing you were looking for is probably in there somewhere. I keep hoping that one day I’ll come in and there will be a proper system, but I know it will never happen.”

Does this not sound like the sad truth of existence? Is the disarray of Voltaire And Rousseau not more fundamentally honest than the high street bookshops, which blatantly lie to us that the cosmos can be neatly and alphabetically organised?

As local novelist Louise Welsh once said of the McGonigles’ “poor filing system”: “It can be frustrating, but it does encourage serendipity.” Welsh lives just around the corner, and used to run a semi-rival dealership nearby, which closed more than a decade ago. She has since had more success writing books than she ever had selling them – her best-selling 2002 debut, The Cutting Room, was partly set in this little antiquarian district on the left bank of the Kelvin. But Eddie McGonigle still gives her a discount, and sympathises with any neighbour or shopkeeper who has to pay rent at West End prices. His older brother Joe has owned this property outright since the mid-1970s.

“That’s probably why we’re still going,” he says. It also helps explain how they can afford to offer most of their fiction for £1, and a tiny, leather-bound, 19th-century copy of The Iliad for “about £4.50”. He recalls one customer being so insulted to see how little resale value they attached to her favourite author (poet, essayist and gardening writer Beverley Nichols) that she stormed out.

Earlier this month, the Provincial Booksellers Fairs Association announced its support for more than 600 independent retailers who say they can’t compete with Oxfam’s growing chain of city bookshops, which benefit from the breaks that come with charity status.

Eddie can see both sides – “some second-hand places are so highly priced that I don’t know how they make a penny” – but Voltaire And Rousseau is still cheaper than the Oxfam on Glasgow’s Byres Road. “We could probably stay open even if we weren’t selling at all.”

Working with family, of course, is a pretty sure way of keeping your overheads down. Their father came to Glasgow from Strabane in Northern Ireland, their mother from Iona. The way Eddie tells it, Gerry was always “the big reader” of his nine siblings, and the first man in the city to recognise a market for selling used books by post in the late 1960s.

Well, maybe the second. “I think there was also an Asian gentleman in Cowcaddens,” he says, “but hardly anyone else was doing it, so thousands of books were just ­pouring out of these old tenements.” It was Joe who had the idea for the original Park Road branch of Voltaire And Rousseau, just over the Kelvin river – although Gerry came up with the name – and they drafted in the other McGonigle children to help build it. Eddie, one of the youngest, sawed the wood for ­the shelves.

At some point there was a falling out between the older brothers. Gerry went his own way, and eventually retired. Joe has run the shop since it moved to Otago Lane, but stays mostly behind the scenes.

“Joe is more into the business side of things,” says Eddie. “He’s not much of a reader, except for stuff about financial crises. His ideal book would be something on the Wall Street crash of 1929.” I would like to ask Joe how much the current recession has hurt or helped his profits, but he appears only briefly today, and seems genuinely shy – certainly much quieter than his little dog Fifi, who sleeps on the books when she isn’t barking. Meanwhile, Eddie minds the store.

He once left to sell school textbooks, and again to train as a mental health nurse, “but that felt like an awful responsibility. I know it seems like a long way from this job, but we do get some guys who come in here and tell us that they haven’t spoken to anyone else in a week.”

They also get the occasional “specialist”, such as “Jiminy Fish”, so nicknamed by Eddie for his single-minded interest in the literature of angling. They get the odd celebrity too. The painter and author Alasdair Gray is such a frequent patron that he has written the shop into his own fiction, some of which can in turn be seen poking out from between the masses of old and newish novels. The Reverend Ian Paisley occasionally pops in for theology books, as he did once on Eddie’s day off, when he cheekily asked after “the old boy who usually sits behind the counter”. (Paisley, Eddie points out, is at least 25 years his senior.)

And then there is a man called Bob, who has come in three times a week since the day the shop opened almost 40 years ago. “Bob’s life is books,” says Eddie. “The ­problem is that he’s got no room left at home for any more, and his wife keeps asking him to have a clear-out. He’s reluctant. It’s like giving up something he loves.” Some of the McGonigles’ oldest customers are now passing away, and their collections are coming back to Voltaire And Rousseau.

“The relatives might want rid of them, or the deceased might have left them to us. We’ve done a lot of those big house calls recently. Vast amounts of books. Joe had to do a few trips in the van.” It’s not fanciful, then, to imagine that there are lifetimes worth of reading in here. All the dust on the covers, the past owners’ names on the inside pages, their hand-written notes in the margins, create the impression that the shop itself is sentient, and the books seem to know what you’re thinking: you will never have enough time to read them all, even if you had the space to store them.

Personally, I have worried about this since I was a child. As soon as I was told that I would die one day, I started reading each new book with an underlying panic, but my pleasure deepened too.

I think Marcel Proust once said something about the days we lived most fully as children being the ones we spent with our favourite books. If I can’t quite remember the exact quote, it might be because I have never actually read his magnum opus, In Search Of Lost Time, although I always promised myself I would. And here it is, of course, just above head height, a stack within a stack – all three volumes in mint condition. “How much?” I ask. “£10 for the set,” says Joe McGonigle, who has just replaced Eddie at the counter without me even noticing. Obviously, I must buy them. These books are monumental.

If they fell on you, they would kill you. But while I am digging them out, the thought does occur to me that this is a room I wouldn’t mind dying in.