Neil Gaimen's "Sandman" series has long since transcended the comic book genre (it was the 1st monthly comic to win a literary award)and earned a cult following of loyal fans who find added enjoyment with repeated readings. "The Sandman Companion" caters to those fans.
*What it is - There are excellent summaries of each of the ten graphic novels, sidebars pointing out details not to miss, black and white sketches scattered throughout, commentary by the artists who drew the series, and in-depth interviews with Neil Gaimen on each book. Gaimen, in particular, is very insightful about the creative process, including the Alan Moore-like research he put into the backround of many of the stories. All of these were not only fascinating to read, but also gave added depth and perspective on future readings.
*What it is not - "The Sandman Companion" does not clear up backstory, fill in plot ambiguities, nor spell out what is only hinted or implied in the books. It stays fairly true to what you are given on the printed page, and from that standpoint, there is nothing "new" in this book - its all about what already IS. So if you are one who dislikes the amibiguities or open-endedness of some parts of the plot, you won't find much help in this book.
I'd recommend this for fans of the Sandman series who simply want a glimpse of what went on behind the creative process of one of the greatest comics in history.