The Clash always liked dressing up. In her 1980 book The Clash: Before & After, rock photographer Pennie Smith captured the period when their artily stencilled punk gear was gradually being relegated to the back of the wardrobe. The band discovered fedoras and posed like Depression-era gangsters or Brighton Rock spivs. Bassist Paul Simonon cut an unforgettable dash in biker-gang gear that could have come straight from Marlon Brando’s The Wild One.

Musically, they were undergoing a similar transformation. But one of the main points Marcus Gray makes in this book is that rather than branching out into uncharted territory, the eclecticism The Clash demonstrated on their third LP was actually a return to their roots.

Gray has already written a biography of the band, Last Gang In Town, but in Route 19 Revisited he concentrates exclusively on the epoch-making London Calling, their third and greatest long-player, an album which, 30 years after its release, is routinely cited as one of the greatest ever made. To do that he goes back to the beginning, long before the punks declared 1976 their Year Zero, when the future Clash frontman, Joe Strummer, was still an unknown R&B buff who idolised Woody Guthrie and claimed that most of his favourite records had been made before 1965. He was yet to meet aspiring guitarist Mick Jones, an aficionado of sixties rock who had soaked up music at concerts in Hyde Park and still held a torch for Mott The Hoople. Paul Simonon had been a teenage skinhead and never lost his fondness for ska and reggae, while drummer Topper Headon, the band’s most accomplished musician, could easily jam his way through a jazz set if required.

The role of flagship punk band suited The Clash at first, not least because, with the exception of Headon, who once auditioned for Sparks and reached the final two, they couldn’t play well enough to emulate their heroes. As they grew in skill and confidence and became more keenly aware of the limitations of being punk figureheads, the tastes of their formative years reasserted themselves.

London Calling was the first reconnection between punk and earlier music forms, a declaration to the punks that their music could break out of its stylistic straitjacket without turning into mainstream, radio-friendly dross. It was also the record on which the band who had famously sung I’m So Bored With The USA consummated their love affair with America. Gray calls it the invention of “post-punk roots music”, and it was topped off with a photograph of Paul Simonon trashing his bass, again taken by Pennie Smith, which practically redefined the word “iconic”.

If Gray doesn’t reveal everything you might want to know about the making of London Calling, he dredges up enough detail to satisfy all but the most obsessive, including microphone placement in the studio, basslines which were double-tracked by Jones, and which song their alcoholic maverick producer Guy Stevens attempted to spice up by flinging plastic chairs around the studio. All good stuff.

But one of his undeclared aims seems to be a reassessment of the lyrical talents of the late, much-missed Joe Strummer. Gray lauds Strummer as a Dylanesque writer with switched-on antennae who took events from his own life and those of the wider world and, in writing sessions in his “spliffbunker” (a little den in the studio constructed from instrument cases) condensed them into compact, resonant rock’n’roll lyrics. In this lair he joined the dots between outlaw ballads and old ska singles (not forgetting Paul Simonon’s pantheon of big-screen heroes) to perfect The Clash’s patent brand of rebel rock.

Gray’s track-by-track analysis is the centrepiece of his book, greatly assisted by the availability of Strummer’s old notebooks, which show how his lyrics evolved. We now know, for instance, that the title track started off life as a diatribe against plastic mods streaming into the capital looking for Swinging London (the line “we ain’t got no swing” survived to the final version) until Jones recognised their backing track was too mighty to bear so petty a lyric and encouraged him to play up the apocalyptic imagery instead.

Gray reserves his greatest praise for the ambitious lyric of Spanish Bombs, the complexity of which he partly attributes to Strummer’s attempts to slip in a romantic theme under cover of something more macho. Strummer, it seems, was never comfortable exposing his softer side and frequently donated songs like Lost In The Supermarket to the reedier-voiced Mick Jones, in order to “distance himself from his own sensitivity”.

As befits a band which would baffle its audience with the kaleidoscopic triple-album Sandinista! the following year, a thorough examination of the songs throws up such diverse topics as a potted history of the rude boy, a brisk outline of the Basque separatist movement, ancient Chinese erotic texts, 1950s movie star Montgomery Clift and American folk song hero Stagger Lee. Even the Knights Templar get a namecheck, although that might be Marcus Gray’s attempt to match his near-namesake Greil Marcus for pushing cultural connections to the limit.

Like Ian MacDonald’s excellent Beatles analysis, Revolution In The Head, Gray’s book is a triumph in that his obsessive detail enhances and illuminates a classic record rather than demystifying and diminishing it. It’s big and occasionally rambling, like the album that inspired it, but every bit as hard to ignore. Now let’s see one on Sandinista!

Route 19 Revisited by Marcus Gray is published by Jonathan Cape, priced £20.