Mental Ray Bible,
This review is from: Mental Ray for Maya, 3ds Max, and XSI: A 3D Artist's Guide to Rendering (Paperback)
This book is excellant.
I already had a solid understanding of Mental ray but this book supplied me with a greater picture and understanding of the inner workings of Mental Ray, something i would not have been able to achieve with any number of interactive tutorials. Mental Ray is explained from the base up covering several advanced topics.
Buy this book if you'd like to know almost everything about mental ray there is to know....