In particular- when I am not reading metaphysics books or introspection based books, I would have to say that the Warhammer 40k books are AMAZING. Graham McNeill completely brings the Ultramarine's chapter to life as well as the forces of Chaos when they are the subject.

Another good series of the Warhammer 40k is books based on the Imperial Guard- the Ciaphas Cain series in particular- the dude is a hero in my heart.