This is a good companion novella for someone who is obsessed with the series. It reads like a guide to the characters, followed by short backstories about many of them before, during, and after the flash.
I really enjoyed the short stories, because I am fascinated by the characters.
If you are not a fan, this is obviously not for you. Make sure you don’t read it until after book 3. It can be read between books 3 & 4, which was the intention. It can also be read after book 4, if you somehow skipped it.