This isn't your usual ground troop battle. Sure, there is some of that, but the big focus is the Titans, the gigantic war machines. If you have no idea what a Titan is, think along the lines of MechWarrior; they're gigantic, vaguely human shaped tanks which can deal out unimaginable destruction. Unlike the Battletech mechs, Titans are not controlled by one pilot sitting in a chair, they're so big and complex they have a crew and are piloted by a dedicated "captain" who is permanently encased in liquid filled pod so he can be connected to a machine and experience everything that the machine experiences. This setup improves his performance by making it an extension of himself, however also opens his body to physical manifestations of injuries the machine takes as well as creating a separation between him and his crew.
The story centers around an attack on a forge world. The plant has been pumping out weapons for some crusade that is being waged by the Imperium. However, the general in charge of that war has pushed too far ahead and taken too many of the forge's Titans for his war, leaving the forge world a relatively small defense force, ripe for the forces of Chaos to attack. Fortunately, a battle group of Titans en route to the front lines is able to divert and render assistance. I'd go into more detail, but it would introduce spoilers. I can tell you that this is the only book I've read so far that shows the friction that underlies the relationship between the Mechanus (Mars) and the Imperium (Earth). It was fascinating to get a picture of what actually holds the empire together and how fragile the relationship actually is.
One of the strengths and weaknesses of the story is that it covers multiple fronts of this war. You are dealing with the namesake of the book, the Titans obviously, but you've also got side plots running involving the support forces for the Titans, the Mechanus working to provide intelligence on the enemy Titans, the survivors of a destroyed tank squad, the remnants of essentially a National Guard deployment, and to lesser extents a brain damaged gardener, a toy maker, the husband of one of those National Guard members, and the Imperial Governor's office. It jumps around a fair amount, making it all a lot to keep track of as you're reading.
I wasn't familiar with the author before this book, but looking around Dan Abnett comes extremely well recommended by reviewers on Amazon. I can see why. While the story wasn't the best I've ever read, the writing was just superb. There are details to print a vivid picture, a bit of levity here and there, characters you can actually care about, and enough suspense to keep you reading. I loved the last Warhammer book I read (Cadian Blood), but I'd read maybe 20 minutes, do something else, come back and read some more. With this book, I started reading and when I stopped it was over an hour later. I was so taken with the writing style that before I'd even finished this one, I ordered one of the Warhammer Omnibus books he's written.
The ending is pretty weak, just sort of trailing off like the author ran out of room, but the book is satisfyingly long, weighing in at a little over 600 pages and well worth the price. It's almost required to include this, so here goes. If you don't know at all what Warhammer 40k is, if you've never read any other books, or if you hate everything about the BattleTech style mech fighting with a vengeance, then this book will probably not be what you're looking for. Even if you're like me and you've read a few of the other books and kind of like it, you'll be searching online a lot for what certain terms mean or for more background on things, though you really can figure out just enough of what things are by context to still follow and enjoy the story. I'd strongly suggest giving the Warhammer Dawn Of War real-time strategy game for PC a try. It's probably the best crash course in the story that you can get without getting into playing the miniatures game.