Expert opinion is divided as to the lowest point of William Shatner's career. Was it his 1976 film, The Devil's Rain, in which he was sacrificed by a bunch of Satanists led by Ernest Borgnine? Or what about White Comanche, where he played twin half-breed brothers who must fight to the death, and which can today be bought on Amazon for one cent? “Literally, they are selling it for one cent,” writes Shatner in his new autobiography, amazed and appalled. The prize must surely go to Incubus, a “metaphysical witchcraft picture” about a beautiful succubus who destroys men's souls; it has the distinction of being the only film to be shot in Esperanto. Sadly, a mistake in the lab destroyed all negatives of the film, although it is available, Shatner informs us, “on DVD at Shatner.com. for $9.95 - That's $2 less than Amazon!”

Never trust a man who uses exclamation marks - they're the prose equivalent of canned laughter, signalling looming comic intent together with a fierce distrust that we will not get the joke. Shatner gets the joke. Shatner is the joke. “I now have 53,038 friends on my MySpace page!” he boasts, like a one-man Trekkie convention, a walking encyclopedia of all things Shatner. He may even be the first celebrity to be found guilty of stalking himself. Up Till Now follows him from his childhood in Jewish Montreal, through the ranks of Canadian repertory theatre, to Stratford for a bit of Shakespeare, then back to New York for the “Golden Age of Television”, where he first learnt to deploy halting cod-Shakespearian diction,putting emphasis. In the. Strangest. Places. In order. To steal scenes. Without anyone. Noticing.

It was only when transplanted to the outer reaches of pulp that this manner really found its raison d'être: if you're going to chew the scenery, the scenery might as well be fake platinum. Shatner was third choice to play Captain Kirk in Star Trek, and he seems to have spent most of that show's three seasons seething with jealousy for his co-stars - “I was supposed to be the star but Leonard Nimoy was getting more attention than I was.” Years later, Nichelle Nichols (Uhura) told Shatner how “self-absorbed” he had been during the series, and how resented he was by the rest of the cast. Officer Sulu hated his guts. Scotty refused to speak to him. It's strangely gratifying to know that a series founded on the idea of inter-galactic friendship was such a bear pit of bruised egos and pointy-ear lashings.

Shatner doesn't let the charge of self-absorption delay him long; there's his appearance on the World Wide Wrestling channel to be getting on with, or the time he sold his kidney stone on eBay... The reader is left to decide whether this is all a sign of incipient postmodernism (the first actor to display knowledge of his own cheesiness) or just an ego so hungry that no crumb is too small to be worth chasing under the table. He did do all this stuff, after all. “I've been an answer on Jeopardy but I've never appeared on the Home Shopping Network,” he writes. It's also debatable whether we ever get to know the man behind the desperado shtick. It comes as a shock when, at one point, Shatner describes himself as “a divorced father of three in the back of a truck”. Hang on: a divorce! And three children! Where did they come from? Either he has found a way to self-reproduce, like Tribbles, or he is not the fondest of family chroniclers.

He has already written a much better autobiography: the album Has Been, on which, with the help of the musician Ben Folds and the writer Nick Hornby, he perfected the sorry-lounge-singer routine he's been working on since the 1970s; the lyrics, largely autobiographical, have all the sadness and honesty and self-reflection that are missing here.

One night, driving home after promoting the album, Shatner turned on the local radio station and was overjoyed to find it being played. “We've got William Shatner's new record here,” said the DJ. “Yeah what an asshole,” said a co-host. “You're right, he probably is an asshole.” Shatner immediately dialled the station. “This is William Shatner here and I am not an asshole,” he protested. “Would an asshole call a radio station to complain that he is not an asshole?” Five minutes of wooing and wrangling later, he hung up and turned the radio back on. “Was that really William Shatner?” “Yeah it was. And he's still an asshole.”

Has Been made number two in Billboard's Top Heatseekers chart. No exclamation mark.

Up Till Now by William Shatner with David Fisher
Macmillan £18.99 pp358 Buy from Books First £17.09 with free delivery

Times Recommends

• Superfreakonomics by Steven D Levitt and Stephen J Dubner
• The Times picks the 50 best paperbacks of 2009
• Vincent van Gogh’s letters show the soul of the artist