Post Office, Ham on Rye, Women and Factotum are all REALLY good novels. Buc is a love or hate kinda writer. Really gruff, vulgar, and plain but funny as hell and about the most truthful gutsy novelist I've ever read. Lots of whores, sex, TONS of alcohol. The man drank like an absolute fish. Did I mention alcohol? And whores?
His poetry was his bread and butter for the most of his career, and just about all of it is worth picking up if you like it. I love everything he's done, but it's not for everybody.
If you have to get a new copy, most barnes and nobels sell his books. You can either read the novels in the order he wrote them, or in cronology of his life (they're all really autobiographies, though they're certainly embellished with a bit of fiction here and there). If you want to start with his first novel, got with Post Office. It will show you where he began as a novelist. If you want to go with the life cronology, start with Ham on Rye. It's probably my favorite.
Enjoy him, and make sure to have plenty of wine, whiskey, and cheap beer on hand while reading to get you in the mood. If it seems too vulgar or alcohol soaked, drink until it's funny!