Aaron Dembski-Bowden's one of the better authors writing for Games Workshop's Black Library. Yep, that's as nerdy as it sounds.
Black Legion didn't disappoint, following on from The Talon of Horus which I finished late last year, the book continues the plot and tells the story of the Black Legion's founding.
For those who aren't massive nerds, the Black Legion are 40k's bad guys, led by this angry looking fella. Despite being about the bad guys, the writing is fun and engaging in ways few other 40k books can match.
A fun reads for fans of the franchise, and quite possibly one of the better entry points into the story if you're interested in understanding what 40k is all about. Certainly a better entry point than the never ending Horus Heresy series.