Inherent Vice might be a good choice. (Certainly, it's a better choice than Gravity's Rainbow, anyway.)
It's a goofy romp of a novel about a stoner private investigator in 1970 Southern California named Doc Sportello. Doc has to investigate the disappearance of his ex-girlfriend Shasta, and her new boyfriend, a real estate mogul named Mickey Wolfmann. Along the way, he has to avoid running afoul of the hippie-hating, straight-laced LAPD detective Bigfoot Bjornsen, who is also investigating the case.
But then things get a little odd — there's an organization called Golden Fang that is made up of dentists may be a drug front for...something else. There are beach bunnies, and threesomes, and LSD trips, and an awesomely hilarious trip to Vegas, and some crooked cops, and a hitman, and man, this novel was just a lot of fun.
The dialogue in this novel is freakin' fantastic — it contains a lot of Pynchon's signature jokes and puns. (Guy says, in reference to evidence, "we'll have to run this by the lab," calls his dog, a Labrador comes bounding in, etc.) It's stupid, but it's funny. Beyond the jokes, though, there are plenty of digressions, and things just happen for no particular reason — you won't not confuse this with a Pynchon novel, even if the "psychedelic noir" is a new genre for Tommy P. And just when you start think you've got it figured out, you veer off into another direction. It's definitely not just a straightforward mystery, but that is part of the fun of the novel.
And I really did have fun with this book. It's one of those novels that, if you don't let yourself get frustrated that you don't get every joke and movie and music reference (you won't) or remember every tenuous character connection (you won't), and just enjoy the plot and writing, you'll probably have fun with it, too.
Also of note, director Paul Thomas Anderson (of The Master, There Will Be Blood, and Boogie Nights fame) has just wrapped filming on an adaptation of Inherent Vice. It's the first time a Pynchon novel has been adapted for the big screen. And it's expected to be released mid- to late-2014. I wrote a post for Book Riot about the adaptation today, which I'll post here later this week, as well.