SSH Mastery: OpenSSH, PuTTY, Tunnels and Keys
on Feb. 15, 2012
Great little book about SSH - even though the name includes mastery it contains the things you should know if you work with OpenSSH.
I like the tone and writing style, and also specifically books that won't just tell you about features to use, but also have the guts to say "this feature is not to be used, never" and then won't even include it - root logins are bad, period!