Grey Knights by Ben Counter shows the effort these warriors will go to while trying to foil the plans of the demon Ghargatuloth. Following a trail that leads to the planets in the Trail of St Evisser they find a myriad of forces arrayed against them. The Grey Knights soon find themselves fighting for the very survival of the universe.
Counter's fast paced writing style pushes the story along, without pausing to catch its breath. This is a great read that provides a lot of information about these elite warriors of the Warhammer 40K universe.