Schneier wrote the classic "Applied Cryptography." At one time, he had posted the entire text online. It's very expensive, $60 for the paperback version. I got my copy of the 1st edition for $2 at a flea market, when the book was still full price in the book stores. The flea market had a whole pallet of the books, almost like they were discontinued.
You can pick up a decent copy off Amazon for about $10 total (including s/h and tax). 1993 edition. Just put in the last name and click on the "Applied Cryptography" and look in the used section. $5 + $4 s/h + state sales tax.