Ascendance continues the story right from where "Sacraments of Fire" left off, so you should have read at least that one before starting this.
The focus here lies on finishing up the Ascendants-storyline that was begun during the pre-Destiny relaunch books and picked up in Sacraments. Now we know what happened with Taran'atar and a bunch of Ascendants... and, although the Ascendants' attack takes place 6 years in the past, it still comes full circle in the end. I guess this is one of the perks of this story, its construction because the parts which seemed quite without connection, the attack and the events at the scientific research outpost suddenly come together. Otherwise, while I quite enjoyed the one-on-one scenes with Ro and Taran'atar and Kira and Ascendant respectively, this part of the story didn't touch me all that much - but I don't blame that on the book itself, but rather that I don't have the background of having read the earlier parts of the relaunch where the Ascendants and Ghemor first appeared.
The other, to me more interesting part of Sacraments, namely the upheaval after the discovery of alien technology on one of Bajor's moon and its ramifications for Bajor were unfortunately a bit sidelined. At least marooned Altek Dans finally is allowed to actually step foot on Bajor - after becoming romantically involved with Ro (who hasn't told Quark yet - cowardice? Or trying to keep a fall-back option?)... and this is threatening to become an interesting triangle with the final paragraph of this book. But I really don't understand the investigation into Altek's story. Given that he hails from the distant past, from a time where Bajor wasn't yet united, isn't it possible that the names of places could have changed? Instead of just reiterating that there are no places with the names Altek gave on Bajor... why not try to narrow the search by looking for landmarks or approaching the issue via geographical site rather than name. For instance my home town had quite a few different names throughout history, so why shouldn't that be the case for Bajoran cities as well? And speaking of which: If Altek studied Bajor and DS9's history, did he never come across a picture of Kira, his lover in the distant past?
Overall, much of Ascendance feels like tying up loose ends, and it was good to see Captain Kira and Commander Vaughn again as the leading officers of DS9, or get a glimpse of Vic Fontaine (although I hope he just remains a background story, and won't get a book of his own. I'm not that interested in Fontaine after all - although I appreciated the nod to The Light Fantastic). Much of Ascendance also deals with life changing decisions, such as Dax's being restless on DS9 (and malcontent and all-knowing after one course on advanced tactics...) and eventually pursuing a command for herself, such as Kira joining the clergy, or Odo deciding to return to the Gamma Quadrant (albeit at a time before Kira's return), even Taran'atar and the Ascendants finding peace and a purpose. Although each of these character-moments could have been fleshed out more. They feel a bit squeezed in among all the war-mongering of the Ascendants and jumping between the different locations, more like an afterthought of putting to paper what had already been inferred in previous books (especially concerning Dax who we know assumes command of the Aventine eventually, and Kira). I quite like the status quo on the station now, I like the new crew, and definitely liked that Blackmer was appointed First Officer. But I have to admit, save for Blackmer the other characters do need some background as well. But for those little character moments, however, Ascendance was a rather average book.
Fortunately, there's still much to look forward to, upheaval on Bajor, Sisko's mission into the Gamma Quadrant, Odo's wish to return to the Founders... a bright new future ahead for DS9, after the, I guess, necessary clean up of past events. Bring it on.