The books I recommend include:
Gunter Osterloh's "Leica M" - but this book is out of print, and not easy to find, sometimes you can find it on ebay. It's my favourite book. But it is not cheap now. Some have described it as the best book on Leica RF.
Jonathan Eastland's "Leica M compendium"
Ghester Sartorius' "Identifying Leica Lenses"
For Digital M, the book to look for is "Leica M digital photography" by Brian Bower
The big bookstores here do not stock much on RF. Riceball is your best bet - if you want it here and now.:bsmilie: I'm assuming you are in Singapore. The other place to go is Amazon.com