In the current New York Review of Books, Charles Rosen has a masterful review of W. A. Mozart by Hermann Abert, which he succinctly calls the Best Book on Mozart. (There's also a fine and revealing study of the literature on Gertrude Bell, written by Rory Stewart who recently stood in her shoes).
Abert managed to set down practically everything of interest about Mozart's life that was known in 1919, and he added a complete overview of Mozart's works, very many of them discussed in great detail and related to a masterly account of the music world in Mozart's time and the different musical traditions of the age.
My current concern is how I should approach my "new" Mozart works. In college, I realized that I tended to listen to a recording over and over, until I had worn all the edges off the piece and knew what was coming, and then somehow the work lost its voice. And so I saved the operas on purpose, and the quintets by accident. But there's a limit to these things: I tend to save stuff for rainy days until it becomes stale and mouldy.
Incidentally, I think saving music was a dumb idea. Use it: there's plenty more.
In the event, though, it seems to me that I ought to be reading something about these Mozart pieces — something not too long (because I really don't have time to undertake 1,500 pages of Mozart even if I could follow it) and that would, ideally, open new vistas. Something that will keep me from knocking the wearing these out. Ideas? Email me.
Update: Dave Phillips and Andreas Vlach both recommend David Hurwitz, Getting the Most out of Mozart