The Flash: The Secret Files of Barry Allen
This was okay for what it was: A guide of the show aimed at younger readers. It offers an overview of the show, brief descriptions of the major characters, and an episode guide for seasons 1-4.
However, the organization was haphazard at best, AND they kept mixing up the various Harrison Wells doppelgangers. For example: Using a picture of Earth 2's Harrison "Harry" Wells on the character profile of Earth 19's Harrison "H.R." Wells. I know they all look alike, and I know they're all played by Tom Cavanagh, but they are not interchangeable. That kind of mix-up should not happen in an official book.
Also, if you already didn't know, Harrison Wells (in pretty much all flavors) is my favorite character, so that really didn't sit well with me.