A Fine Line Between Stupid and Clever – credited to the late Rob Reiner, with Christopher Guest, Michael McKean and Harry Shearer, as well as to their Tap alter-egos Marty Dibergi, Nigel Tufnel, David St Hubbins and Derek Smalls – serves as a fitting companion to This is Spinal Tap (1984), the mother of all mockumentaries, much beloved by middle-aged men and their poor put-upon children. (My wife and my daughter, I should say, absolutely hate it: but then they prefer Pitch Perfect – and Pitch Perfect 2. So there’s no accounting for taste.)

Part oral history, part behind-the-scenes memoir and part self-aware parody of rock memoir, the book’s a bit of a mess – much like the fictional band. It could well have been a work of serious cultural criticism if written by, say, a David Hepworth or a Greil Marcus, a proper rock critic, examining the context of Spinal Tap’s emergence in relation to late-stage capitalism and what happens when a parody becomes canonical and… Actually, I’ve lost interest in completing that sentence, so who the hell would want to read a whole book like that?

For those who somehow missed the original film and the recent sequel, Spinal Tap II: The End Continues, the premise is simple. A fictional British heavy metal band blunder their way through a disastrous US tour, satirising the pomposity and excess of rock stardom. The film has long since entered the comic pantheon, its faux-documentary style endlessly imitated. Without Spinal Tap would we have The Office or, indeed, Alan Partridge? And would that be a loss? (All Souls Fellowship exam question-setters, please note.)

A Fine Line promises to tell the full Spinal Tap story – how the film was made, why it matters, how the band evolved – and it sort of does and definitely doesn’t. The book’s chief pleasure is the wealth – the excess, the superabundance – of anecdote. We learn how they came up with the outfits, the accents (Christopher Guest’s father was English), the music, the style; how the first cut of the film ran to four hours; how the test screenings in Dallas and Seattle were near-disasters; how Guest, McKean and Shearer liked to stay in character – which is all very interesting showbiz-type stuff, if not exactly revelatory.

What’s really interesting, indeed extraordinary, is how a film that was shot in five weeks for just more than $2 million, with improvised dialogue and no real stars – Anjelica Huston and Billy Crystal’s memorable cameos excepted – and championed by a studio that had absolutely no idea what to do with it, managed to become a slow burn word-of-mouth success with such a long and strange afterlife. The fictional band became a real band, touring, recording and developing a genuine fanbase: they exist because audiences refused to let them remain imaginary.

This is hardly unique. The Bubba Gump Shrimp Company, I believe, though I’ve never had the pleasure of dining at one of its establishments, is an actual seafood restaurant chain inspired by the film Forrest Gump; you can buy Homer Simpson’s favourite Duff Beer; real-life Quidditch exists, etc. There’s probably something very profound, or at least very pretentious, to say about all this, which may be related to what Ludwig Feuerbach says about the nature of religious belief and Slavoj Zizek’s ideas about fetishistic disavowal. Fortunately, the book does not say anything about any of that. It does, however, explain how the Hollywood studios managed to screw the creative team out of any money, and how it took them years to reach a settlement and claim back their intellectual property rights.

For devotees, therefore, the book is pretty much the perfect read. But anyone else should be aware that a certain level of familiarity with the film is assumed, without which the whole enterprise might seem simply baffling and self-indulgent. The multiple voices – Reiner’s and those of the three leads – don’t resolve into a single, clear story and the timeline and tone slip back and forth between nostalgia, digression and, frankly, filler. There’s a chapter, for example, in which ‘rock musicians’ recount ‘the effect the film had on them’. Steve and Steve from Toto recall watching the film and ‘dying with laughter’ – not literally; and Aimee Mann tells some stories about her own Tap-like flat meat and mini-bread issues while touring with Hall & Oates. Hilarious. But probably not for everyone.

In the end, A Fine Line Between Stupid and Clever lives up to its name. I found it genial, funny and rather encouraging: an invented band become their own legend. And I am, of course, contractually obliged to end this review by saying something like ‘After all these years, they’re still going to eleven.’