Shearer’s research, we discover, included tour time with British metal warriors Saxon. It was Guest, in a Greenwich Village guitar store, who made a mental note of the wasted rocker with a migrating baguette down his pants. In their touring comedy-trio days, an incompetent promo guy really did bend over and plead, “Kick my ass. I’m not asking you, I’m telling you: Kick my ass.”

Forty years after his film was savaged by test audiences and buried by uncomprehending studio and marketing heads, Reiner is naturally delighted to know it’s not only on permanent loop in every self-respecting rock band’s tour bus, it’s in the US Library of Congress Film Registry alongside Citizen Kane and Casablanca. But as their wigs turn grey and lycra waistbands tighten, the ongoing activity of Spinal Tap, the band, is the most remarkable proof of the stupid/clever coin-toss. Tap has toured the world, playing their expanding repertoire of hits – Sex Farm, Bitch School, Big Bottom, Break Like the Wind – from Carnegie and Royal Albert halls to Glastonbury.

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Pimped up like rock’n’roll Halloween, they’ve shared stages and studios with bona fide metal monsters in awe of their legend. The great Jeff Beck, far from sulking over that stolen haircut, sought Guest out on a trip to LA and gave him a guitar. Wait, two guitars. One got nicked in the lobby. In a scene ostensibly obsessed with authenticity, what’s most twisted is how Tap keep so real. “The joke band is the only one that’s playing live,” one observer mentioned to Shearer on a 2001 arena date. “Because by that point, nearly everyone was singing to [backing] tracks.

“We still get, ‘I didn’t realise you guys can actually play’,” Guest says with a note of irritation, possibly wondering how many “real” guitar heroes have even glimpsed Woody Guthrie’s mandolin in a museum case. An appendix entry of sheet music for his romantic piano adagio Lick My Love Pump (in D minor) proves his commitment to both music and parody.

And just when you thought you had that fine line sussed, it turns upside down. Flip A Fine Line Between Stupid and Clever over, and it becomes Smell The Book: The Oral History of Spinal Tap by Reiner’s in-film alter-ego Marty DiBergi, with St Hubbins, Smalls and Tufnel as co-authors, and a foreword by the actual David Byrne.

It’s a shorter but no less enlightening read, a physical gag baked into the spine of the book, and one more reminder that however seriously you take your rock’n’roll, it’s only a pratfall away from ridiculous.

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