I was hoping that the Sandman Companion wouldn't be too much of a repeat of the Sandman Annotations online, and, happily, it isn't at all. There's definitely enough new stuff to make it worth the cover price. Besides that, some things many fans are already familiar with from rumors and/or interviews are clarified here, like the origin of Death, and the connection between Delirium and Tori Amos.
Each Sandman collection is discussed, one at a time, with insightful essays by Hy Bender, and then the Neil Gaiman interviews. There really is a lot of new information. For every issue, almost, there's some neat fact you didn't know before, or a comment from Neil. (For example, it's no coincidence that "Three Septembers and a January" sounds like "Four Weddings and a Funeral", although the comic was released before the movie!) Even if I had known everything in this book already, Neil's interviews are always fun to read.
The book includes an excellent introduction and "Frequently Asked Questions", with questions such as "Why should I read a comic book?" The answer to this question is the perfect thing to show someone who turns their nose up at comics. Yes, comics can be worthy literature, and not only that, they have potential for artistic effects that can't be achieved through film or words alone.
The Sandman Companion is also well illustrated. With the text, there are black-and-white illustrations by Sandman artists, including panels from the comic, trading card art, and proposal sketches--early ideas for what Dream should look like. Then there's a great full-color section with more trading cards, and cool stuff like Sandman statues and posters (Now that I own picures of them, I don't feel so bad about not being able to get my hands on the actual items), foreign-language editions of the comic, and even some of Dave McKean's Vertigo Tarot (Mad Hettie as The High Priestess, Dream as The Hierophant, Delirium as Temperance, Destiny as The Wheel of Fortune, and Death as...well, Death).
This is a very well-done companion book to the Sandman series, readable and constantly interesting. I really recommend it. The hardcover price may be a bit much for the casual fan, but any serious Gaiman fanatic (like me!) should buy this as soon as possible. I'd suggest that after you've finished the Sandman series at least once, read the Companion as you re-read each volume. That's what I'm doing--it's like discovering the series all over again.