Where do you usually get your books?
Being one of those who lives life not by plan but by random chance and opportunity, I tend to find mine in the 25- or 50-cent bins of used bookstores.
I tend to buy hardcover books, never paperbacks; it's perhaps silly to be this way, but one can be a snob about such things. I grew up in houses with floor-to-ceiling bookshelves lined with slipcased books (the Heritage Club, the Folio Society, &c.), and I always had the impression if a book was any good, it was good enough to be in hardcover.
Of course, buying from 25- and 50-cent book bins means one is reading old or outdated books, but knowledge is eternal. Here we are in 2009, and at the moment most of the books I'm buying are from the late 1970s, early 1980s. About 90% of them ultimately go to the thrift store, but there have been some real keepers.