Indeed, this is the darkest incarceration of Star Trek I can remember reading and it does so without the "Space Whale" elements of armies of cyborg zombies or scary dogmatic aliens who hate humans. Instead, what makes Vanguard dark and personal is the human stories (even for the aliens) which are going on. The space station Vanguard is a place where the humans of Roddenberry's enlightened future are nasty, meanspirited, lost, confused, and full of all too believable emotional trauma. The fact Kirk, Uhura, and Spock find the place deeply unpleasant during their short visit also nicely establishes it's not a revisionist take on the Federation but it's THIS SPECIFIC PLACE which is the Mos Eisley of their territory.
HARBINGER works essentially like an anthology of collected stories. We have Commodore Reyes, who is the fuming admiral who clearly hates his position on the Vanguard despite the fact he has convinced himself it's of vital Federation importance. We have T'Prynn, a Vulcan intelligence agent whose life was ruined by the culture which raised her. She's also a lesbian or bisexual character in literature which is still catching up for the show. Then there's my favorite character of Pennington, who is a crusading reporter in an adulterous affair that both seem like are things which shouldn't be needed in the 23rd century. The only character I really didn't warm to was Cervantes Quinn who seems a bit like a much-less competent Han Solo (or a much-more competent Harry Mudd) wandered into the Trek universe.
The premise of the first book is Starfleet has constructed Vanguard to secure their position against Klingon and Tholian Space (or possibly something bigger). While sending out one of their ships, it gets destroyed by the Tholians and a government cover-up is enacted to prevent war with the former. Because if the Federation goes to war with the Tholians, they'll be easy pickings for the Klingons and vice versa. This ties into everyone else's story because Quinn stole the materials which the starship was replacing, Pennington is lovers with one of the dead crewmen, and Reyes is the architect of the cover-up with T'Prynn as his agent.
I found T'Prynn the most interesting of the characters and enjoyable even though she seemed the most like a book character. I love the idea of a Vulcan who has just been utterly screwed over by her culture from birth until present. I also like the fact she's an amateur jazz piano player because that's just a character quirk which says so much about the character. Watching her handle Reyes' dirty work while maintaining her Vulcan stoicism worked well.
I also very much like the dualistic cowardly and crusading elements of Pennington. In the 21st century, being exposed as an adulterer is bad but hardly life-destroying but he's terrified of it and goes to elaborate lengths to cover it up. He also is outraged by Starfleet's various actions and cover-ups but is less concerned about actually revealing the truth than the "Big Scoop." He's a mass of contradictions and I like the implications he's just a terrible selfish person in his marriage but an excellent journalist. Still, it makes you wonder what Orianna (his lover) ever saw in him.
This feels very much like the set up for a new series with original characters and very little in the way of guest stars from the canon. There's a few but they're not central. Instead, the book asks us to go on a ride with an entirely new set of heroes and a tone much darker and more "realistic" than Trek normally goes. Much to my surprise, I'm entirely onboard and fascinated with the stories of its flawed and human cast (even the nonhumans).
In conclusion, I really loved this book and will be picking up the other installments of the series. I like the handling of Starfleet and the Federation's politics in this time period. It's a little more off-beat and less utopian but not so much as to be unrecognizable. I think the troubled protagonists really bring out the heart of this story and the darker, more militaristic realpolitic motivated Starfleet causes the story to have more stakes. You don't think these guys will do the right thing and yet when they don't, it's still surprising.