We hosted a book swap / potluck today, and it was fantastic. Might I recommend that you do the same? We've been doing some rearranging, tidying, and cleaning out of our books, and we discovered that there were 123 books in this house that we didn't feel the need to keep. So we invited a pile of friends over, asked them to bring any books they no longer wanted, and it was a free-for-all.
Phillip did a great job at evaluating his books and deciding which to keep and which to give away. (That picture was taken before anyone came - that's only some of the books that we contributed.) He ended up donating about 1/3 of his collection, which was many, many books. And in return, we picked some of the ones that were brought by others, and the boys have some new-to-them books to enjoy. The left over books will be donated to the library.
Even better than that, we had piles of friends in the house for the afternoon. I loooooove having people over. Now that the kids are for the most part old enough to more-or-less play on their own together, there's an extra level of fun. I can't think of a better way to spend the day.