The Twofish Encryption Algorithm: A 128–Bit Block Cipher Hardcover – Import, 14 April 1999

Bruce Schneier is the Yoda of crpytography and information security, if you've heard him there's no need to ask anyone else. This book would probably only appeal to the genuinely avid crypto fan, its Greek to anyone else. It is a thorough exposition of the making of the Twofish cypher, the latest attempt to secure information as absolutely as possible. You must be well versed in cryptography to get anything out of the work.