In my comments on Gods of Night I made a few guesses about what might be coming. I was wrong about a couple of things, most notably that the flashbacks were mostly over and done with, and that Voyager might have more of a role in this one. But we do get to know Erika Hernandez a lot better in this book, as her storyline becomes almost entirely about her and how she copes with isolation, loss, aging, and her change into a new form of life. She continues to be a sympathetic character, trying to find the right thing to do and trying to keep from giving in to despair or madness. The longer she stays there, and the more she learns about the Caeliar, the more humanlike their behaviour seems to be. There are petty and paranoid leaders, but there are also characters like Inyx, who demonstrate compassion, friendship, and scientific curiosity. (It's not unusual in stories about long-lived beings from alien civilizations to portray them as having abandoned their interest in progress and science, becoming decadent over the millennia.)
The other storylines progress quite well, though David Mack's not going to lose that Angel of Death tag by writing books like this. It's good to see things coming together, as the Caeliar and the subspace tunnels in the Azure Nebula turn out to be connected. The glimpses of what was at the other side of the tunnels were intriguing. The ancient galaxy with stars hidden behind shells had me expecting something very bad. Worf was right to ask whether they really wanted to get the attention of the builders of those shells. (Are they the Caeliar who went far back in time?)
It was ironic that the big epic battle for the Enterprise and the Aventine in this book turns out to involve the Hirogen rather than the Borg. But it's a very different kind of fight than they would have had with the Borg, and allows for a more suspenseful shipboard battle (and the EVA element was a nice touch). Two ships slugging away at each other are never as interesting as two characters slugging away at each other.
Speaking of Voyager (well, by way of the Hirogen), I'm not sure what's going on with Seven of Nine in this story. She doesn't seem to be entirely herself. I don't think it's just me, because her scene with Jellico was picked for the inside-the-front-cover excerpt. Is there something going on with her? Will that have some kind of payoff in the third book, or is it tied in with something in Kirsten Beyer's books, or am I making too much out of it?
Speaking of acting in an unusual manner, Deanna Troi seems to have gone off the deep end in this one. Her reactions seem to be purely emotional. She's always been a strongly emotional character but she's rarely seemed self-destructively stupid. At least we're given the impression that she's hiding her situation successfully from most of the Titan crew members with her, so the reader can't get too frustrated with the other characters not acting on a problem they don't know about. Doctor Ree's solution is certainly unexpected, though. I thought after the first book that Caeliar tech might offer some magical solution for Troi's problem, but the revelation that Hernandez's continued existence is based on catoms and requires staying near the Caeliar (or does it? she'll find out soon enough) made that seem less likely. Ree's intervention makes it seem even less likely, though it'll presumably force Troi to accept Caeliar medical help. I'd be very surprised if Ree has ended up killing the patient in order to save her.
Where the first book offered a variety of story types in its alternating sections, this one has two main stories playing out: the ever more violent struggle of the Federation to survive the Borg attack, and Hernandez's much quieter struggle to cope with her polite incarceration among the Caeliar. The Titan storyline this time seems to be more about putting pieces in place and building up to the meeting with the Caeliar, though it had its share of good character moments, building Ra-Havreii and Melora Pazlar's characters in particular. I liked the fact that a seemingly sensible high tech solution to Pazlar's problems is actually rooted in Ra-Havreii's psychological and emotional issues.
Hernandez's story is much smaller and more intimate than the Borg epic, but the emotional weight is often much stronger. I can't imagine what it'd be like to experience something like a Borg invasion, but being alone in a place you don't want to be, watching people close to you die, those kinds of things anyone should be able to relate to. And given how complete Hernandez's isolation is, and how long it lasts, those emotional stakes keep getting raised. And meanwhile, though Nan Bacco et al back on Earth don't get many scenes, they're good ones. Using the Ferengi to get the Breen onside as mercenaries was a nice touch, not that it ultimately made much difference at the nebula.
The line I never thought I'd see in a Star Trek book: "I want a hard shag, and I don't care who knows it!" (And the ferret line on the previous page.) Funny and unexpected but also very real, considering the circumstances.
All in all the second book doesn't look like it'll feel like a case of middle volume syndrome when the trilogy's complete; a lot happens, and some things are resolved (the story of Hernandez's last several hundred years, the failed attempt to block the Borg at the nebula). There's a lot of intimate drama and all-out action, and certainly no shortage of suspense. The intensity increased from first book to second, and will doubtless do the same in the third. And though we've been warned and shown that everything is wide open now that the 24th century is almost the exclusive property of the books, I have a strong feeling that the third book will have several "I can't believe they did that without a reset button" moments. I don't even want to try to predict what's going to happen next, because I just don't know how far David Mack et al. are willing to go.
In summary: this trilogy combines everything that's great about the last few years' worth of Star Trek novels: Titan's sense of wonder; TNG's political intrigues; DS9's example of creating new characters alongside familiar faces, who quickly become as essential to the stories in their own right as the old stalwarts; SCE's focus on building and developing new characters and making drastic changes in their lives, or ending them; and the wide open possibilities of a Star Trek no longer constrained by what might happen in next week's TV episode. It's a mature and powerful work that would surprise the hell out of people who are dismissive of Star Trek novels, if they'd give it a chance.