There is a problem in that most of the standard books are now a few years old, and therefore miss out the most recent developments. As there have been more developments in the last 6 year or so than in the preceding 16, that's a pity. However, i would suggest having a look at one of the following:-
First two from the 1990's:
a) The Leica M Compendium, Jonathan Eastland - dates from 1994, thus goes as far as the M6, but obviously nothing later. Nearly 200 pages, and gets into 'Leica philosophy' as well as just the facts;
b) Leica M Photography, Brian Bower - dates from 1995 so the same time-frame as the Eastland book. I prefer this one; it was my first Leica 'read'. It's short than the Eastland book but (IMHO) is better illustrated. In colour, too.
Now two from the early noughties:
c) Leica M Advanced Photo School, Gunter Osterloh - dates from 2005 so is the most up-to-date. It's comprehensive & recent; the only trouble is that it's really a tutorial about photography with a Leica (as it's title suggests) and the overview information about hardware is a bit sparse. I suspect that it's really intended for an audience that already knows it.
d) Rangefinder - equipment, history, techniques. Dates from 2003 and is written by Roger Hicks and Frances Schultz. Roger posts here quite frequently. It's a good overview of the whole rangefinder scene, and as such goes much wider that simply Leica.
My personal preference would be b) and d) as introductions, then perhaps a) and finally c). But that's just my opinion, of course.