This book has the lowdown on virtually every Leica M and R body and lens ever made - starting with the Ur Leica of 1913 -18!  It's a mine of information, giving production variations, manufacturing dates, filter sizes, technical data (for example, MTF curves for lenses), and serial numbers related to the year of manufacture.  Buying secondhand Leica?  You must have this book!      7th Edition; published Oct 2002. 280 pages.