This article from Curbed SF mentions Prisoners Literature Project as a good place to donate your unwanted books!

https://sf.curbed.com/2019/1/24/18182230/donate-books-clothes-furniture-cleaning-marie-kondo-where

(We dig this a lot, but reminder: please try to taylor physical book donations to those on our donation list – https://www.prisonlit.org/donate/ – tho we’re also looking for sci-fi and fantasy paperbacks recently.)