Independent book sellers. Sounds weird, why would they give books away for free? I have a friend who sells books online and buys huge lots of books to resell, many of which are not resellable due to condition or oversaturation in the market. I have gotten a truly obscene amount of free books from her because of this. Lots of sellers have books they want to unload and often have to resort to throwing them away.

be advised that these programs are often co-opted by local officials to boost numbers and reduce actual available services to incarcerated populations, or to deflect blame for employee drug smuggling

your experience will vary depending on the county

its wise to partner with an existing, experienced program first to learn the field a bit and avoid the pitfalls counties lay for beginners

for example, many counties will use bureaucratic expense approval hurdles or materials clearance and transportation hurdles to slow down and exhaust your efforts, but there are people with the specific experience and networks to navigate that stuff already in some cases

Prison Book Program https://prisonbookprogram.org/ & Books to prisoners https://www.bookstoprisoners.net/

A local or regional library often (but not always) serves jails in their community already. If not, they may be open to extending operations there. If that fails, libraries often rotate out stock to make room for newer, or more popular books. Anything they dispose of would be older, but for this situation, that may not be as much of an issue.

Project Gutenberg, if ebooks

Anna's Ar-- Oh, nvm.

Give more info about location and language, then maybe people can help better.

Also maybe asking in the local community for book donations might be an option.

Just put a sign out front and a box.

I want to add that I’ve heard you can get cheaper postal rates if it’s a book.

When I hear people talking about books, they often speak of them as if it was a comodity. Do some people just read a random book every once in a while?

What am I missing?