You could donate them to the library, instead, perhaps? If they were interested in adding them to the collection, it would leave the possibility of reading them in the future, wtihout having to keep them in the meanwhile. (I have sometimes asked my library if they will add a donation to the collection, specifying that I will only donate the book if they will; they seem to be fine with that. Of course, the book in question was a new young adult fantasy by a popular author, and they didn't have a copy yet.)