Priests of Mars is the first book in a recent trilogy about an Adeptus Mechanicus search of an old magos, Telok, who seems found some ancient marvel but was considered lost. A Rogue Trader has a device that can pinpoint the last known location of the magos, so an expedition of cog servants, Cadian imperial guard troops and Black Templars space marines sets sail to find it, with a small fleet of spaceships led by the Speranza, an ancient and valued Ark Mechanicus type vessel.
I had never read much about the Adeptus Mechanicus except some basic lore and some mentions other books do to them, so I'm quite pleased to find they are quite interesting indeed. I know understand the desire of people for Games Workshop (or should I say Forge World?) to bring them as a full Warhammer 40,000 army, they are a dark version of the classic sci-fi cyborg, mixed in the grim scenario of the 41st millennium.
The book is well written, showcases multiple character groups (humans, mechanicus, space marines, even xenos), the pacing is good and thrilling, and although of couse the story halts with a nice cliffhanger, it hook me enough to already have started reading the second book. Definetly recommended for WH40k fans.