I got most of my "library" from the Half Price book stores in Austin while living in Round Rock. Good find.
I am a HUGE fan of used books.
A fraction of the price of a new book, Unless there is something horribly wrong with it, like missing pages or water damage, a used book is every bit as useful as a new book.
It allows something that would have been discarded or abandoned to continue to be useful.
It is the finest form of recycling.