This was the required text for two of my graduate-level Antenna Theory courses, and it is a very comprehensive and detailed text. The book assumes a basic electromagnetic background (gained from a previous undergraduate e-mag course, and the author also writes a book on the subject). While the text may seem daunting to the sheer size and thickness of the book, it is very easy to read and follow along. The book also comes with a CD of lecture notes in PDF format, along with animations and MATLAB scripts that help in the understanding of the material. The lecture notes summarize the important equations and pictures in the text and are of great assistance.
Overall, this is one of the better engineering textbooks I have encountered, and it will have a permanent place on my bookshelf. If you are interested in gaining a solid background in antenna theory and design, look no further than this book!