Shortly before Christmas a trailer was posted online for The Hobbit: An Unexpected Journey, the first part of Peter Jackson's long-awaited film of JRR Tolkien's early novel.

It's due out next December, and the sequel, The Hobbit: There And Back Again, a year after that. Insiders say it shows Jackson back on top form, though as I've never seen his The Lord Of The Rings trilogy, I couldn't comment, even if I were to watch the film. Barring being marched to the cinema at gun-point, I can safely say I never will.

It's not that the little I've glimpsed of it isn't appealing. There's a stout and younger Bilbo Baggins (Martin Freeman), and Ian McKellar as the craggy Gandalf, exuding a warm, but serious twinkle. The set is brighter than anything real life could match, there's lots of olde-worlde English-style countryside standing in for the Shire, and a posse of dwarves so puckishly individual they make Snow White's companions look like clones.

The problem for me is that films of Tolkien's books trample on my child's-eye view of his world. Unlike human literary characters, most of Tolkien's creations cannot be properly replicated on screen, since they do not exist except in the author's and his readers' imagination. Hobbits are not simply diminutive humans. They're something entirely different. So too Gollum, and Shelob, and the elves in Lothlorien who, once committed to a physical shape on celluloid, lose some of the alarm or magic they conjure when only made of words.

I ought, I suppose, to be grateful that Jackson has turned to The Hobbit, since to my mind it is Tolkien's finest work. The fanfare around The Lord Of The Rings has overshadowed the merits of his first novel, which is a far simpler and more rollicking adventure, though not without moments of terror. I still remember shivering when, for instance, the dwarves have been trussed up in the treetops by gigantic spiders looking for a good meal.

"The meat's alive and kicking," says one, as fat little Bombur kicks his captor, who has pinched his nose because he looks juicy. "I'll soon put an end to that," hissed the angry spider ...

Thereafter I kept a sharp eye on the spiders in our water-tank cupboard in the eaves. Now I knew what they were capable of.

I may be unfairly prejudging Jackson's film from a fleeting preview, but it seems to me that he hints at the darkness of the later trilogy by casting a portentous shadow on a story that should have nothing glowering over it except Gandalf's eyebrows. When originally conceived The Hobbit, published in 1937, was a gem of children's fiction: beautifully written, imaginatively and amusingly plotted, perfectly paced, and almost flawless in execution. By comparison, The Lord Of The Rings, for all its emotional and political depth, is rawer and less polished.

Much has been made of the influence of Tolkien's experiences in the First World War on his late masterpiece. In contrast, The Hobbit was almost as blithe an exorcism as one could imagine of the horrors of war. It was a bachelor's story, about the particular kind of comradeship possible between a band of men or, in this case, a hobbit, 13 dwarves and a wizard.

Only after his publisher Allen and Unwin asked for a sequel did Tolkien consider embarking on a more ambitious book. No-one could have dreamed The Hobbit would be followed by such a magnificent, if uneven work. When Tolkien put aside childish things to create his magnum opus, he secured his place in the annals of adult literature. Yet with his juvenile novel, he had already stamped an indelible footprint in the world of children's books. The Hobbit ranks among the finest, and could arguably be considered the very best children's book ever written. Jackson has turned to it last, but for me and I suspect countless others, it is a story we met at a young and impressionable age, and it will always be first.