Teleread has an interesting post entitled How to give away an ebook after you’ve read it in which the author suggests people purchase ebooks for their local library rather for themselves, thus allowing multiple people to read the book (once the original purchaser has had a chance to read it). Sounds like a great idea, except, as folks in the comments point out, most licenses won't allow for this. But there are some ideas in there that, in a sane world, could work. Alas.