Supposedly female authors aren't as funny as male authors. That's probably true when by "funny" you mean "good at writing sexist jokes".
So I had a quick think about which books and authors make me laugh-out-loud. Generally, I don't turn to fiction for the LOLs, but the books that do make me laugh also tend to be ones that I read over and over. Bridget Jones' Diary has given me fits of giggles in all manner of embarrassing public places. I have indeed laughed out loud at Jane Austen, although her wry humour tends to be more of the "quiet smile" type, rather than "huge guffaw". Likewise Steve Martin, whose novels are surprisingly not in-your-face but instead quietly funny. Some of Nick Earls' work is like that, and some of it is rolling-about-on-the-floor material. (Worryingly, the most hilarious scenes always involve cats.) Jaclyn Moriarty's YA books are funny, even when ringing painfully true.
One of the funniest novels I have ever read is Kate Atkinson's Emotionally Weird. I love this book to death and laugh every time, although it also seems to get sadder every time -- but sometimes humour is like that. It's hard to find a good piece to quote to convince you, as a lot of the humour involves setups and callbacks. One of her strengths is spearing the foibles of her very real characters, usually by their dress. This is from an early scene in the novel:
Small and thin, Terri was adressed, as usual, in the manner of a deranged Victorian governess. She had the pale pallor of a three-day-old corpse on her cheek and, despite the dark on the unlit stair, was wearing Wayfarer Ray-Bans.
Although I had opened the door, Terri's finger remained on the doorbell, as if she had been struck by rigor mortis while pressing it. I forcibly removed the finger, almost having to break it in the process. She held out a hand, palm up, and said, 'Give me your George Elliot essay,' her face as expressionless as an assassin's.
'Or what -- die?'
'Fuck off, she said succinctly and lit up a cigarette.
Jasper Fforde is good for smug little one-liners, such as, "None of them can tell a Mark Twain from a Samuel Clemens," and Hamlet's coffee shop soliloquoy in Something Rotten is pretty much the best scene in any book, ever. It begins: "To espresso or to latte, that is the question. Whether 'tis tastier on the palate to choose white mocha over plain, or to take a cup to go..."
I could go on: Kurt Vonnegut. Bill Bryson (although he is non-fiction). Douglas Adams. Who's your favourite funny fiction author?