Saying it's your favorite Troy Denning novel may not sound like high praise but you must understand that I know Troy Denning from my childhood onward. I read the entirety of the VERDANT PASSAGE series of Dark Sun (where I fell for Sadira of Tyr as my first fictional crush), every one of seventeen or so Forgotten Realms novels, PAGES OF PAIN for Planescape, and his Star Wars series too. I've read a LOT of Troy Denning novels over the years. This is, undoubtedly, the best.
Part of what makes it so good is the low-key premise. This is a smaller story set in a universe of galaxy-destroying superweapons and hostile alien powers. It's a murder mystery and that already makes it unique among Halo novels. It's still undoubtedly Halo with Fred-104 (a.k.a the second best Spartan-II), a group of Spartan-III soldiers, a Forerunner installation, and even an acknowledgement of the Insurrectionist movement. It's just we have less shooting thousands of Grunts and Elite versus character development. That automatically puts it above most Halo novels and they've done more than most Expanded Universes for fleshing out the lore of the mega sci-fi franchise.
Last Light's premise is an underground hot spring and resort has been taken over by the military to investigate an insane Forerunner A.I. that is floating around it. The UNSC is keeping it a secret, though, because they don't want the local government to potentially seize it for itself. You see, after the destruction of the Covenant--all of humanity's old grievances have come back. Veta Lopis is a cop who hates the UNSC, ONI, and isn't too fond of the Spartans either. However, she's more dedicated to finding out who is responsible for the series of murders going on.
I love Veta Lopis and think she's one of the best Halo characters aside from Cortana, Doctor Halsey, and the Spartans. She plays off of Fred-104 tremendously well. I also like their small amount of sexual tension that Troy plays mostly for humor. The Spartan-IIIs, which I was iffy on until Halo: Reach, are also treated quite well here. The group is really expanded on once they have an older Spartan to serve as their father-figure. The mystery of who is responsible for the murders is never in doubt but what to do about it is the real issue.
I think part of what works about this novel is Troy Denning makes all of the characters (except for the corrupt official Arno Castille) sympathetic. Veta Lopis wants to solve a murder and that trumps her hostility to the Spartans and UNSC. Intrepid Eye is insane from millennium of isolation but the humans are the intruders into his home. Fred-104 is just trying to keep his people safe and a potential galaxy-disturbing artifact out of the hands of a bunch of militant revolutionaries. Even the Keepers of the One Freedom are fairly affable Covenant religious fanatics. I am probably more sympathetic to them than I should be since they're not interested in genocide of the human race.
The action in this book is great and there's even a few ridiculously fun moments like Veta using Fred as a sled down a snowy mountainside due to his armor locking up. She's embarrassed when she finds out he was conscious during the whole ordeal and even makes a quip, ("Next time, I'm on top."). I also like the confusion over whether one of the Spartan-IIIs could have done the murder even though we know they haven't since some of them have actual psychotic rages if they're off their meds too long. Great job, ONI. Real smooth move there.
I feel like also pointing out it's nice to have Troy Denning make the people of Gao descendants of non-United States or European humans. They have a Chinese-named world and are mostly descendants of South American people. It affects their personalities, accents, and behavior in a way that makes the world feel a little larger. It's a small detail but one I appreciate from the author.
In short, I loved this book and I think anyone who loves Halo will enjoy it. It's even an enjoyable piece of military science fiction for someone who isn't familiar with the franchise. I listened to the audiobook version of this book and prefer it to the physical version as the narration is top notch. If Halo novels were more like this one versus some of the more recent ones (which shall remain nameless), I'd still be as big a fan of them as I was years ago.