I agree with sinbad.
Fundamentals of Power Electronics by Erickson and Maksimovic
This book is very solid and nearly covers all necessary knowledge in power electronics.
The authors composed this book with a very easy-to-read and easy-to-understand fashion, and the equations and graphs are also clear. They also spent good portions in concept explanations.
I knew that some USA universities also refer to Mohan's Power Electronics. This book is also great!