I received Companion to Narnia, as well as a boxed set of the Chronicles themselves, as a birthday gift. I must say it was one of the best gifts I have ever received.
I initially read the Chronicles as a young child, then again in Jr. High. I'm afraid I let my passion for the stories slip through my high school years, just when I needed them most, but now, after my third year of college, I have renewed my interest in the works of Mr. Lewis.
The Companion is easy to use, extremely informative, and actually fun to read straight through, even without using it as a reference. The entries are encyclopedia-style, alphabetically listed, with the page numbers the entry appears on in the Chronicles, related entries, and often very extensive footnotes referring the reader to books and articles by Mr. Lewis and other renowned Fanstasy and Religious authors, as well as background into what specific uses could have meant to the author, or his inspirations for them.
There are annexes at the end on everything to how old the Pevensie children and others are in the books, Narnia time vs Earth time, the age of Lewis during the writing of the Chronicles, maps of Narnia and more. Wonderful illustrations are distributed throughout the book, just waiting to surprise you when you turn the page.
I would HIGHLY recommend this book to any lover of Narnia, whether it's your first time around or your fortieth. A lot of questions I remember having when I was younger are answered, and you gain a deeper meaning of what it "behind everyone's story".