I'm in the same boat. I'd be happy to give books away to people locally (I'm in Los Angeles) but I'm kinda too busy to deal with the post office. There's a cool web site or two that address this problem (a friend of mine is a big fan of one which I believe is called PaperbookBookSwap.com or something like that), but it's not worth it to me to wait at the post office. (Waiting in lines drives me nuts.)

In the past I've given books away to the local library. Where I live now, they only take donations on Saturday mornings, so I've just been giving them to Goodwill instead. There's something about donating a Haskell intro to Goodwill that fills very stupid, though. I hope they have some system where they put them all on eBay or sell them upstream somewhere to some kind of distributor, but I think they just throw them on the shelves.

If we were to build or find some kind of "free programmer books" site where we could just co-ordinate giveaways, I'd be very happy to kick in a ton of books, and probably to keep doing so for years.