The Ultimate Death Note Encyclopedia!
Death Note 13: How to Read, also known as Death Note: How to Read 13, is an encyclopedia-style guide to the Death Note manga series, including character bios, storyline summaries, interviews with creators Tsugumi Ohba and Takeshi Obata, production notes and commentaries and a bonus: the pilot chapter of the manga. It also includes Ryuk's Human Observation Journal.
Everything contained within this book is official, written by the original author and artist, and is considered canon for the manga series.