Before I began reading this book, I was informed that it was probably the "most sci-fi" out of all the books in the Aurora Rhapsody series. I was also told it wasn't going to just be Abysm II; it would be a new chapter so to speak, with a different tone and style (which, if you've read the rest of the series, you may have been able to predict based on Abysm's ending).
Both of these things turned out to be 100% accurate.
While I'll always have a soft spot for Vertigo and Transcendence, I think Relativity has become a major contender for fave-of-the-series. Abysm dropped a major bombshell on us there at the end, and Relativity jumps straight into the action with Alex and Caleb acting on humanity's new-found knowledge. The book's blurb says it all: the humans aren't going to just stand by and let the enemy come to them this time. They're planning on beating the Anadens to the punch, and to do so they'll need help from characters both new and familiar.
Re/Genesis, a short story set just prior to the events of Relativity (I highly recommend reading it before this book), gives us a thrilling introduction to Eren asi-Idoni, one of those new characters. While I'm sure he'd be plenty capable of handling himself in combat situations, his strong suits are wit, agility, and all-out insanity. A fresh face amid a sea of characters who feel like old friends by this point, Eren makes a formidable -- if chaotic -- ally for our heroes, all while dealing with problems of his own and trying to escape from his past. Like the book itself, he has quickly become one of my favorite characters. He grew so much in this story alone, and I can't wait to see what the remainder of the series holds for him.
In addition to facing the possibility of an Anaden invasion, many of the cast members find themselves targeted by an unknown enemy, one whose attacks are up close and personal, in more ways than one. Fan-favorites like Devon, Mia, Malcolm, and Brooklyn end up working together in ways we haven't seen yet in this series, drawing on new technology and experiences in order to protect themselves and each other. Meanwhile, Miriam is as strong and stoic as ever as she leads humanity into the unknown.
I mentioned that this was the most sci fi book out of the entire series. That's not to say the others somehow aren't; this one just contains SO MUCH material. In the parts of the story that take place within Aurora, the cyberpunk element is more dominant than ever before, possibly because there are more Prevos/Artificials than ever before, and they're constantly evolving. New bleeding-edge technology ends up playing a crucial role in a sweeping space battle, the likes of which we haven't seen since Transcendence. The "military sci-fi" aspect of this series gets a chance to shine here. Meanwhile in Amaranthe, the exploration theme that was so dominant in the Aurora Renegades trilogy still plays a large role in this story as Alex and Caleb learn more about the Anadens, the Metigens/Katasketousya, and the dozens of other species who inhabit this alternate universe. Tack on a resistance-versus-evil-empire theme à la Star Wars, a (somewhat comedic) heist element, and some fantastical new alien abilities, and you have perhaps the most action-packed, intriguing, fun, and unique book in the series.
With only two books left (that we know of, anyway!) in this epic tale, it will be interesting to see how the story concludes. I can't wait to find out.