I like this book because you get to know the history of the Continuum (the society of these immortal beings), which is something they didn't have time for in the regular series. Secondly, Q must face another being as powerful as himself and much more evil. This being lives beyond the great barrier because he was banished there to keep him away from any civilization. Here is the link to the book on Amazon. (The paperbacks are the cheapest.)
If you're wondering who Q is or just miss him....here is a video.