Whether your a UNIX system administrator, user or enthusiast, you've probably used OpenSSH at least once in your life. SSH Mastery provides lots of good information for beginner and seasoned ssh users.
SSH Mastery also presents a lot of cool OpenSSH functionality without a lot of filler text. In around 150 pages, Mr. Lucas even manages to provide instructions for those users stuck on Windows.
This is my new goto reference of all things OpenSSH.