I was reluctant to send these book through the Post Office, it being a government agency itself. But then my wife pointed out that they lose money on every item they handle, so we could consider it our own covert "John Galt" action to help further bankrupt the government. Well, that works for me! :-)
Another option is to order the books individually from bookstores on the web and have them drop-shipped directly to the the recipients. With most online retailers, you can include a message with the books, but this is generally restricted to a few hundred characters, so it must be kept short. If you go this route, you can still send a more detailed letter to the politicians later. There are some examples of short messages posted on the Letters Page.
As reported elsewhere, the owner of The Book Resource, a mail order bookstore, has made an offer on the Suggestions Page (suggestion #8) to sell the book for $6.75, and, for an additional $3.60, will ship the book to your recipient with a letter that you email to them. For more information, click here.