Applied Cryptography: Protocols, Algorithms, and Source Code in C, 2nd Edition, by Bruce Schneier. A cryptographic tour de force. This book is packed with useful algorithms and fascinating history covering all of modern cryptography. One-way hash functions, digital signatures, pseudo-random sequence generation, fair coin flips, digital cash, anonymous broadcasting, key management, public-key encryption, and many more. The appendices include complete source code for 9 algorithms including DES and BLOWFISH. The source code is in C but it's worth translating into Visual Basic. In my library, this book has more pages flagged with sticky notes than all my other books combined. [Amazon] [Amazon.co.uk]

The Twofish Encryption Algorithm: A 128-Bit Block Cipher, by Bruce Schneier, John Kelsey, Doug Whiting, David Wagner, Niels Ferguson. 208 pages, paperback, $49.99. Twofish is a symmetric block cipher that takes up to 256 bit keys. It is one of the encryption algorithms being considered by the National Institute of Science and Technology (NIST) as a replacement for DES. This book assumes you know something about cryptography. Read the previous book before you tackle this one. [Amazon] [Amazon.co.uk]

Cryptonomicon, by Neal Stephenson. Fiction. 918 pages, $12.80. A remarkable book tracking parallel cryptographic story lines through World War II and the present. Extremely compelling. [Amazon] [Amazon.co.uk]