Ebook: Foundations of Electromagnetic Theory

I'm writing this review to bolster the review written by glires. I believe that glires's review is very accurate. This book assumes the reader already has a working knowledge of electromagnetism.
This is not a book for self-study. If you want a self study book (or a light reference), I would recommend using David J. Griffiths book or some other book for introductory EMAG. (The book I originally learned from is no longer in print so I can't really recommend it).
That being said, once you get into chapter 3, the book is excellent. This book also uses the paradigm that solving problems is how you learn the theory. So, as glires noted, much of the proofing is left for you to do in the problems at the end of each chapter.
Also as glires noted, this is not a useful reference. There is no single location where all constants and formulas are listed.