Das Chinesische Mah-Jongg-Spiel by Uwe Martens 2005, Books on Demand GmbH, Norderstedt. This book is an excellent German reference containing everything you want to know about the rules used in Germany and many other aspects of the game.
The Complete Book of Mah-Jongg by A. D. Millington, 1977 (with several paperback editions up to 2003), Weidenfeld & Nicolson, London. This is the ultimate reference for Classical Chinese Mah Jong as played in the Western world. It can give you valuable hints about game strategies.
Mah-Jong by Gwyn Headley and Yvonne Seeley, 3rd edition, 2008, A & C Black Publishers Ltd., London. This book is a very easy to use reference, concentrating on the rules as used in Great Britain, defined by the BMJA (The British Mah Jong Association).