This is the fourth book in Marko Kloos's "Frontlines" series. It's become one of my favorite military SF series, as it follows a lot of the tropes without plumbing all the cliches (though it does hit a few of them).
Andrew Grayson, who joined the colonial military in book one to escape a life of crime, violence, and poverty in the dystopian urban zones of 22nd century Earth is slowing climbing the ranks, but at the start of book four, he's still a mere platoon sergeant. Maybe by the end of the series he'll wind up admiral or President, but given all his resistance to even accepting a commission, that would be a bit of a twist. (Then again, when he claims early in this book that he will never, ever become an officer nohow, noway, you can of course be assured that by the end of the book, he's going to become an officer.)
As You Know Bob, humanity has been under assault by the "Lankies," giant Kaiju-like aliens with hitherto-indestructible "seed ships" who drop onto human colony worlds and begin transforming the atmosphere into something unbreathable. For the past three volumes, the war has been mostly humans vs. Lankies, with occasional human-vs-human action (in this far future, the North American Commonwealth is rivals with the Sino-Russia Alliance). Last book, the President of the NAC took half the fleet with all the other high-muckety-mucks and fled Earth, abandoning it to the imminent invasion of the Lankies. The Lankies did not take the Earth homeworld, but they did occupy Mars.
In this book, Grayson is picked by a shady military intelligence officer to join a special mission to go after the NAC leaders and their fleet, with lots of stolen goodies.
Frontlines continues Kloos's adept storytelling and kept me interested, but it was very much a middle volume, with no huge revelations, which really served as an extended chapter in the ongoing story. The Lankies are hardly seen in this book - instead, it's all about the humans trying to unify against the alien threat, requiring that they go unify the hell out of some of their fellow humans, with nukes if need be. Grayson is conflicted about the role he frequently has to play - he does not like being responsible for excessive casualties, his own or that of rival forces. Since his wife and his old Sergeant Major buddy are both along for the ride, I figured we were in for some Significant Deaths. But generally, this all read like a prelude to the next volume, which will clearly be about the combined NAC/SRA fleet trying to take Mars back from the Lankies.
Not a good entry point into the series - start at the beginning. But this one won't let you down, even if it lacked some of the climactic moments of previous volumes.