I use the library or buy used books, for the most part. Buzz and I have a knack for always finding the good used book stores when traveling, and we will hit the local ones from time to time.
For about a year or two, a woman I worked with, and who is also in my book club, volunteered at the local library to take books that were donated but that the library couldn't use. She would gather them all up and donate them to schools or bring them to our school and put them on a Free Book cart in the tutorial center where we worked. After she quit, she asked if she could give me bags of books to bring to the cart in the tutorial. Every month at our book club meetings, she would hand over about 10-12 reusable shopping bags filled with books.
Being who I am, the first thing I would do was to organize and rearrange all the books (invariably, the way I repacked the books, 10-12 bags turned into 6-8 bags).
Of course, if there were books I was interested in, I would just keep them. Over the course of the year or two that we did this, I probably almost doubled my book collection.
I have...a lot of books.