I can often times be a bit leery of media tie-in novels, because they're usually not written by those who write the shows/movies, and that can lead to problems with characterization.
However, this novel written by Susan and Clay Griffith, avoided that.
Barry begins to suffer from what he calls "blurring" during rescues in which he sees an older version of himself and his powers "glitch". These episodes become more and more severe just as a group of villains (Pied Piper, Peekaboo, Weather Wizard, Prism, and The Mist) join forces to wreak havoc on Central City.
Barry, growing more and more weary from the almost constant onslaught of the villains and the blurring (which causes his speed healing to malfunction), calls in The Green Arrow (and Diggle and Felicity) for help.
I really enjoyed this book. For the most part the authors were dead on in regards to the characterization. Not 100%, but pretty darn good all the same.
I also enjoyed the fact that with this being a novel, we got more time to have scenes between characters that we don't normally get in the show. There was a great scene with Iris and Caitlin, for example. And another with Joe and Diggle that I really enjoyed.
The book also spent plenty of time with our villains from their POV, and I absolutely loved the development that Shawna Baez (Peekaboo) got.
As far as where this would fit in the show's timeline, I'd say early season two. After the singularity most definitely, but before Earth-2's Harrison "Harry" Wells shows up. They are aware of Zoom, and I'd say they would have had to have met "Jay Garrick" since Barry knows how to throw lightning in this book, and that's something I was taught by "Jay" (trying so hard not to spoil S2 here). Then again, this isn't exactly show canon, so maybe this is something we're supposed to hand wave, I don't know.
Speaking of Harrison Wells--who is my favorite character, any version of him--I was so disappointed to read in another review that he wasn't in this book. That's not exactly true. While the season 1 version of him isn't a speaking, physical character in the book (due to having been defeated), his presence is hugely felt throughout the entire book, and he is mentioned several times. So, it wasn't as disappointing as it could have been.
I've already preordered the second part of the crossover, Arrow: A Generation of Vipers, and I can't wait to see what happens next.