At Homes For Our Troops their mission is to build specially adapted homes for our severely disabled soldiers and their families. Since they started this organization, they have had an outpouring of support from all across America including monetary donations, donations of building materials and equipment, and fundraisers being hosted in their benefit. http://www.homesforourtroops.org
If you would like to send books to soldiers yourself, check out http://BooksForSoldiers.com where soldiers will post requests for books they'd like to get their hands on and all you have to do is pick a soldier to send a book to.
http://www.anysoldier.com provides soldier contacts, and you read through the names and select the ones you wish to support based on their lists of what they need and want. Any Sodier even has a search capability so you can easily identify specific needs and requests. The soldiers are volunteers for this effort, they see the "ATTN: Any Soldier" line and put your letters and packages into the hands of soldiers who don't get much or any mail.
Through Operation Hero Miles you can donate your airline miles to American soldiers so they can get home to visit with their families. http://www.heromiles.org