The Black Book of Buried Secrets by Rick Riordan

This book is a compendium of information about the 39 Clues that ties up some loose ends from the previous book and is meant to keep readers ready for the next part of the series, which will continue in April and then re-launch in August. If you're obsessive about learning the whole story, you'll enjoy this book, but I wouldn't read it just for its own value.