Inside SSH : from www.macdevcenter.com
Three part article explaining all you really need to know about SSH and then some. If you have time to read only one part, read Part 3. It explains how to use SSH in most secure way.
Wikipedia
Wikipedia article about SSH. Not too technical, but gives bunch of historical information and contains quite a few useful URLs pointing to "related" things.
MAN pages
All you need to know about the world, you can learn from MAN pages. It is not always true, but many Unix devotees do believe it is.
