Have you tried your local thrift store? Maybe librarians might know where to get books cheaply/donations?

First of all, good on you

Would your local independent/radical bookshop be willing to help with some donations or discounts?

Or are ebooks an option? I think verso or other might have options but the may be blocked by fascist prison state

https://booksbythefoot.com/product/boxed-mass-market-paperbacks/

Talk to other libraries

Be sure to make sure they will let you bring them those books too. Some corrective institutions prohibit books unless they come shipped direct from the source (like Amazon for example).

I bet if you put up an ad, on Facebook marketplace or whatever your equivalent is, stating that you are willing to pick up boxes of books (I’d recommend a 3 or 4 box minimum but whatever) people will contact you.

The big libraries here have book sales once per year where you can pay by the box. Might be worth it to touch base with the ones in your area

Contact larger libraries.

I live in NYC and the public libraries here literally throw away tons of books every year. The big city libraries and colleges in your area probably do the same.

Honestly? Go on Facebook marketplace or the buy-nothing groups. Tell them you want free books. People have piles of books in their houses they just don’t care about. I’ve got a ton of kids books I want to give away. And I could part with a bunch of my adult ones too.

But shipping is expensive. So you’ll want to look local.

Thrift shops, yard sales… also local libraries often release their old books when new copies come in, so that’s an angle to investigate…

I used to collect books and the cheapest option at the time was ebay. I would search things like “science fiction bulk” and select the lot with the best titles. Generally I could find some around 1€ per book, don’t know if that’s still plausible.

If your area has them, free libraries. they are these little kiosks whee ueople leave books for others to take. They are usually overflowing.

I would also ask on whatever the equivalent of craigslist in your area. nextdoor, facebook marketplace and free exchange groups.

between all these, I could fill up a van in a day around here

Be sure to check the jail’s donation policy. Many won’t let you just drop off books there. You may have to order the books through specific vendors and they may restrict what types of books are allowed.

Put posters up, to ask the local community, like at the supermarket, if anybody’s got books like that, that they’d be willing to donate.

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Try the dumpster behind your local big box book store. All big stores throw away anything they can't sell

Dont forget to tell Red to get busy living.

I would check libraries. They have to get rid of books all the time as they get old or run out of room for things that are more wanted currently. When book from a popular series comes out or if it becomes popular they get a bunch of copies. Like sometimes as many as a dozen (back in the day at least when physical books were the main thing) but then once most of their patrons have read it and its not as big in demand they drop down to half a dozen. Later still when new crazes have surpassed it maybe 3 and when everyones forgotten about it 1 or worse 0. Of course they will always retain the ones in best condition.

Goodwill outlets or “the bins” (assuming you are in the U.S.). They sell stuff by the pound and often charge less for books. It’s like the last stop for stuff before it goes to a landfill. My husband constantly finds great books there.

Our local libraries have two book sales a month, and give away any you want that don't sell.

Where I live, the libraries have regular sales, but then also the whole library system together will have semiannual book sales at the civic center. Everything is cheap to begin with but then on the last day, you can fill a box for $10 or $20—and the box can be as big as you like, as long as you can get it out of the building. They really need to get rid of the books. You should check with your library and see how they get rid of their culled books.

Pallets, often of mixed books, get sold via auction at the post office reclimation center. You must arrange pickup and shipping. www.govdeals.com/en/asset/145908/4703

Something I really wish I didn’t know about. Hope it’s useful.

Better World Books has donation drop boxes near me.

www.betterworldbooks.com/go/request-books

Make sure the jail will accept the donation before you stock up. Different institutions might have different approved supplier lists because there have been issues with contraband as well as administrators who just like to make having even the simplest of comforts as difficult as possible. Having been a volunteer with the incarcerated, don’t forget that depending on where you live you might have a lot of folks who don’t read English and would appreciate books in their language, and middle school/high school reading level books might be appropriate for some of the folk’s current reading level. I’ve generally found that public, school, and college library discards can be arranged if there’s no middle man handling the books. Even slightly outdated textbooks are useful for people studying for GED or learning ESL.