My guild is one that decided not to keep a library. They brought in books and tried to sell them cheaply - like $1 each - and then offered them for free - and still had books left over. I'm in the same boat as you. My shelves are full, and many of the books are ones that I needed as a new quilter but am no longer interested in. I still buy books though! And a lot of used quilt books on ebay, amazon, and etsy do get sold.
One of my guilds in Oregon had a relationship with a women's prison, teaching inmates how to sew. If there is a prison near you, they might like to have books for their library.
Or rather than throw them away, you could donate them to a thrift store. They might still get thrown away, but at least you will have made the effort.