Death Note was a brilliant series and this encyclopaedia of the series includes everything you would ever want to know about the series, and some things that you didn't. Included in this book are character profiles, plot summaries, interviews with the author and artist, and much, much more.
Some of the information in this book is rather unnecessary and would only be of interest to the most hardcore fan. On the other hand, a lot of the information is very interesting, especially the various interviews which give a nice insight into the processes of making the manga.
As well as this the book also includes the pilot chapter which the Death Note series is based upon and some short four panel Death Note comics that are quite funny. The pilot chapter of Death Note is very interesting and it is fun to see how much changed between that and the series proper, as they are vastly different stories.
Death Note 13: How to Read is a fun book that is a must read for any Death Note fan, although you should read the entire series first as it does include spoilers.