+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.
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!
For detailed information I refer to The Leica: A History Illustrating every Model and Accessory by Paul-Henry van Hasbroeck. Another good resource is Leica Manual by Willard D. Morgan and Henry M. Lester, published over many years in several editions.
well i dont think that you can ever make some rules categorizing by mount. - i declare that i dont own any leica equipment, only soviet stuff -
talking about leica lenses you should know that they change their patents all the time trying to get the 3d-est picture. same labeled lenses differ usually much in practice. earlier m39 lenses dont have coating, or some others have some coating which seems to atract cleaning marks. some of these have been coated afterwards by their owners. other usual problems are haze and fungus in some particular models. anyway some versions like say "red focusing scale" are more sought after by photographers, because they say they have better results than the normal say "black label" lens and the hunt goes on....
Later Ms seem to have abolish the "old look" of m39 lenses that people loved or hated.
gandy's list should educate you, but is it again subjective.
generally today's voightlanders and Ms seem to be closer to the contemporary pictorial standards.
but again i can't find the reason why most leica people buy the expensive leica stuff and dont look on the canon and serenar stuff. i think that most leica people are stuck on the perfectness that leica carries in the name, like the popal what you call it... unmistakeable whatever. even gandy too...i dont think that everything is so perfect anyway
hope i helped. my opinion comes from reading rather than experiencing leica gear. good luck