James Lileks gives some suggestions. The first:
Put them through the return slot at the library and run away. Not recommended.
We at the library had stringent rules on accepting gift books. No-one paid the slightest bit of attention to them.
People got around the rules by dropping off books outside the door of the library in the early hours of the morning. When the staff arrived at nine o'clock to begin their bright and cheery day, there the books would be, like infants abandoned on church steps. We always took pity on them and took them in.
If they were moldy, we got rid of them immediately. Why anyone would think we needed mold at our library when we already had the library board to supply toxicity was not explained. I guess we should have given the moldy books to the board members, but none of them ever opened a book, so that wouldn't work.
Then there were the sets of encyclopedias, many going back as far as the 70's, or more recent ones with one or more volumes missing. And the Reader's digest condensed books, a definite drug on the market.
We also got complete runs of National Geographic, many, many of them. I doubt there is a library in the western hemisphere that doesn't have a complete set of RD since its inception. Many of these were donated to us, over and over. We gave the copies to school kids doing reports so they wouldn't cut the pictures out of our books; I mean our own books, of course.
Most of our donations went into our permanent floating book sale, headquartered in the room with the copy machine, our most popular attraction. The copy machine got heavy use, particularly during tax season, when it usually broke down from overwork.
We got lots of bestsellers, which usually sold rather quickly. Someone once bought Mr Charm a copy of a bestseller--I think it was by Tom Wolfe--and before he could finish it, three copies had been donated to the library. Talk about ausgevorfene gelt!
People donated romance paperbacks, which were added to our collection, ditto paperback mysteries. We put stickers with the date of acquisition on them, and threw them out after a year. These were really popular with the public.
Then once a year, generally in the summer, we would weed, and get rid of duplicate bestsellers, outworn or outdated books, and books that looked so awful no-one would take them out. These included books that we had bound. No-one, we discovered, wants to read library-bound books except a nut like myself, who loves to read neglected and rejected books to see if I can discover a masterpiece. Generally not, but we live in hope.
What surprised me was that people would buy these despised and outcast volumes when they wouldn't take them out of the library for free. The book sale made us hundreds of dollars.
On the principle of waste not want not, we donated a lot of the rejects to the veteran's hospital (the better ones); the county jail (second best); and a pastor in Paterson who would come and get anything that was left (which were awful).
So the public got rid of their unwanted books, we got money for them, and a bunch of people wound up with books they wouldn't have had otherwise, and everybody was happy. Except the library board, of course. But they were never happy, except when they could make life miserable for one or another of the staff.