I'd also recommend two delightful books, both very cheap (£1.00/$2.00) and common on e-bay:
My Way With a Miniature - Lancelot Vining (1941)
35mm Photo Technique - H.S. Newcombe (1945)
Both books are museum pieces, but...
They reflect the experiences of two major photographers from the golden age of the RF and most of the content is still extremely useful, especially for users of Screw Leicas, Contax RFs and their Soviet derivatives. They also inspirational in reminding us of the technical challenges RF photographers faced in the early days. Not to mention beautifully written - especially Vining.
Cheers, Ian