A pair of novels telling an epic tale of the priests of Mars.

SKITARIUS: A discover of ancient technology send and skitarii legion under the command of Haldron-44 Stroika into battle on a world overrun by Chaos. When a cataclysm cuts him off from his tech-priest overseers, Stroika must rally his forces and battle corrupt machines and Chaos Space Marines if he is to achieve victory.

TECH-PRIEST: Magos-Explorator Omnid Torquora of the Cult Mechanicus orchestrates war against the Iron Warriors for control of a long-lost forge world. With mighty Titan Legions, skitarii, and maniples of battle-servitors and robots at his command, victory is within his grasp...until treachery threatens to end his dreams of conquest.

Read it because

It's a truly epic saga that brings all the forces of the Adeptus Mechanicus from the humble Skitarii Rangers to the towering Titan God-Machines into a massive conflict with the fate of worlds in the balance. And there's a scene involving robots and a Thunderhawk that has to be read to be believed.