+1 on the antiquecameras site. The first time I visited it, Google had curiously flagged it as a malicious site but I ignored it and kept reading. I learned the first chunk at cameraquest but antiquecameras went into further detail regarding the various versions of lenses. Before I bought a 50 Summicron, I did my homework and that site helped me out a lot.

Another one that I found very useful over the years is mir.com which can be found at:

He doesn't have a dedicated lens section yet but it is always being updated so perhaps we can expect it in the future. I found it while looking for info on my Contax RTS system and the site has lots of information about it as well including many other makes and models. Happy learning!