I had to think about this topic from Long and Short Reviews, because I've been reading a lot of nonfiction lately, and autobiographies don't always lend themselves to humour. I have been reading Michael McIntyre's two autobiographies which are quite funny, since he apparently uses episodes from his life in his comedy, but I wasn't convinced that counted.
Then I remembered these two.
This Is Going To Hurt and Twas The Nightshift Before Christmas by Adam Kay. Two compilations of stories about his time as a hospital doctor, these are occasionally wince-inducing but mostly hysterically funny. Quite educational, too - I had no idea, for instance, that steroid cream can make the skin thinner, which is not ideal if you're using it as a genital enhancer.
If you haven't read them I definitely recommend getting them this Christmas. You'll collapse laughing.
So what's the funniest book you've ever read?