Erin, I've considered my books, but I have nothing suitable. When we moved in here, I left stacks of books in the Salvation Army bin. I've got an idea, though.
When we've run certain community events at our church, we've approached local stores for donations. Typically, they give a gift card for $25 or whatever. I have't done the approaching, but from what I remember hearing at meetings, a lot of corporations just have a standard amount they'll give in response to a request. Now for us, this has usually been a grocery deal, so we've approached places like Stop & Shop (chain supermarket), BJ's (local discount club akin to Sam's, Costco, etc.).
You might be able to approach Barnes and Noble, Target, Costco and the like, and end up with gift certificates, with which you can purchase more books. The Target corporation has all sorts of community programs, including a grant program: [link]