Titanicus takes a break from the normal lines of battle that make up the wealth of 40k fiction. This story follows Legio Invictus, a Titan Legion thats deployed to Orestus, a vital forge world supporting the Sabbat Crusade (which is pretty much Abnetts Dark Tower at this point) to help defend the planet from a sudden incursion of Chaos Titans and skitarii. This helps to illustrate the dichotomy between Imperial nd Mechanicus, as one seems to have all the power in the universe, and the other is essentially at the mercy of that power.
The storyline fatlers slightly by trying to branch together two separate plotlines, involving a stray squad of auxillary PDF with the Titan Legion engagements. While I'm a fan of ground pounders, and Abnett can write that incredibly well, it was hard to hold the squad members in mind, and I kept losing track of who was doing what why, and who was augmeted and who wasn't. The Titan battles are incredibly well written, and the technology at the disposal of the Mechanicus was both impressive and disheartening, in that its knowledge lost that can't be regained.
The other sideplot, which ties nicely to the recently released HH novels was also a good side tag, that helped to round out the ending, and allowed the Mechanicus characters to display more than their normal level of humanity, although it came dangerously close to tossing GW's holy status quo out the window.
All told, the book is great, and only suffered from one very weak thread. I'd happily heard another installment of Titan warfare, but it would probably start to bleed to Mechwarrior at that point. The God Machines don't work well with others, either. Heres hoping the manage to work it into future HH novels as they did here.