These are the best books on collecting Leica cameras and lenses.
Lager's Illustrated set is the best for photos of the collectible items. Laney's Collector's Guide is the most comprehensive book overall. Van Hasbroeck, in Leica A History Illustrating Every Model and Accessory attempts to classify the different lens variations. The Hove Leica Camera and Lens pocket book has a lot of detail about serial numbers at the end of the book.
All of the other books are for completing the collection if you can't find some information from these main books.