If there are any books that are appropriate (see their donation guidelines) and you can afford the shipping, you might want to consider Ethiopia Reads
- they place books into libraries for Ethiopian children who either have no books in their schools or have no money to go to school. A lot of these kids walk miles to the library just to teach themselves to read.
Half Price books will buy anything they can resell, and will donate out the rest or recycle anything that is totally unusable.
Local libraries usually have a donation station for a yearly book sale, I know ours does.
Our County/City building has a bin where you can donate reading material to prisoners