Mostly I use the library. I read way too much to buy everything and my tastes range all over the place -- I find stuff I didn't even know I wanted to read. our library is great and has or can get most everything I want. However, there are a very few authors whose work I love and know I will read over and over, so I will buy those books. Although I usually wait for the paperback or the local library book sale to pick up a copy for cheap.
I do love our library book sale, though. Once a year the county system cleans house and sells paperbacks for $1 and hardcovers for $4 (prices go down throughout the weekend, too so by Sunday they're giving you all you can cram in a box for $5). Not all books are library books -- many are donations that are in like new condition. Someone bought it, read it once and donated it and the library sells it. So I can collect my favorite authors for very little money there and usually get books in great condition. I have also been known to buy a few books there to salt away in the event of an illness or snowstorm or some other time when I just can't get to the library. It's like an emergency stash. I should put it behind glass with a sign "In case of emergency, break glass."
Otherwise there are some reference books I need for work that I will buy at full price if I have to out of necessity, but those become tax deductions at the end of the year.