The Grey Knights are a special chapter of Space Marines, created to fight with daemons and chaos forces, and led by the Inquisition (they act as the Inquisition military arm).
In this book they will hunt for informaton about how to stop the summoning of a terrible chaos daemon that was banished one thousand years ago and is coming back.
Well written, with a nice plot and good characters, thrilling, and with plot twists. Apart from depicting the GKs as ultra-religious and awesome in combat (as they should be in theory), their foes are nicely presented, detailed and suited for the story told. A good representation of chaos and cultists.
If you want to learn more about the Grey Knights definetly a good action book.