I used to live in an area where the public library would have a book sale every month. You could go and fill up a bag of books for a quarter, I think. You could get old classics for next to nothing, as well. Has anyone found any treasures that way? I mean really valuable books.
I think most Libraries still do that. Depending on which city you live in the price may vary. They also sell or give away magazines. Libraries have to get more involved in community activities since they are now in competition with the internet.
I haven't come across many treasures at library sales as most of those books need to appeal to the wider public, and I have more rarefied tastes. The best books that I have found over the years were hidden in paper boxes at opportunity or thrift shops. It's amazing what people give away, totally unaware of the value of first editions in pristine condition. Even when I alert the staff about my findings, they usually tell me that I am the finder and therefore I can have the book for a couple of dollars.
Most of the books I payed a good price for come from the flea market. The sellers there have all kinds of merchandise and the pricing is very good since most times they have no idea what they're selling Apart from this I caught a few online deals where the books came in packs with free shipping. As for libraries, I have none close to me so I rarely get the chance to visit them.