Doug Rinckes' book covers the electrical systems of the R25, R26 and R27 singles, as well as the R50-69S twins. It includes a great deal of information and procedures, and is heavily illustrated. Full color wiring diagrams are included, as well as specifications and common modifications. It's available from Bench Mark Works. It's softcover, wire bound, has 66 pages and 49 illustrations. Highly recommended!
These reprints cover /2 and R27 technical modifications and recommendations made by the factory as these bikes were improved. Available from Cycle Works, Inc.
All, or nearly all, BMW parts books and owners manuals for pre war bikes are available as reprints (there generally are no separate workshop manuals). Check with Bench Mark Works for a North American source or else contact one of the German vendors. If you have the opportunity to visit one of the big vintage bike flea markets in Germany, such as Veterama, there will be someone there vending these manuals. They typically cost €20-25 each.
