From the reviews: "The main aim of this book is to provide a self-contained introduction to the topic together with a large exposition of the recent results ... . The book is very well written with an interesting level of difficulty which makes it easy to read. It is recommended to everyone interested in this area, beginners and specialists, since it starts with a good introduction and the presentation of rather simple results and finishes with a nice exposition of current research." (Frederic Rousset, Mathematical Reviews, Issue 2007 b)