You get of what you put in. (Sort of)

I had a lot of good books that I finished reading and wouldn't realistically touch again.

Whenever I went to browse for some books I would leave one of them in exchange. Over time, the quality went up because other people started doing the same.

To be honest, I did curate the available books at it as well. Obvious crap (self-published conspiracy theory stuff) was thrown out. At some point you will also have to simple throw out some old ones if they never get taken. Space is limited and a 50 year old book that is collecting dust is not useful to anyone.