SSH, The Secure Shell: The Definitive Guide 2nd (second) Edition by Daniel J. Barrett, Richard E. Silverman, Robert G. Byrnes published by O'Reilly Media
Even though this book was fun to read and contains some interesting ssh trivia. It is starting to show its age in a quite severe way. But if you read around the outdated things, there are quite some good bits in there which are explained quite well.