"The Leica Collector's Guide" by Dennis Laney has just re-emerged in a spanking new edition. I have the first edition of this one, which is excellent.
"50 Years Leica M" by Gunter Osterloh is fairly recent. Parts are somewhat coffee-table, but other parts are very useful indeed IMHO.
Definitely agree with recommendations for the Pocket Book (I have copies of the 1st and current editions), and for Lager's trilogy if you can find them (these also cover the screw cameras and reflexes of course).
If you can find it, Brian Bower's "Leica M Photography" is a good read, as is Jonathan Eastland's Leica M Compendium".
Then there are the relevant sections of "Collecting & Using Classic Cameras" by Ivor Matanle, and our very own Roger Hicks' "History of the 35mm Still Camera", both of which are stalwarts of any respectable library IMVHO.