I haven't really written a book round-up post since finishing Moby Dick last year. It's mostly been Horus Heresy stuff since:
Fear To Tread - The story of the Blood Angels legion being led into a trap early in the heresy timeline. Sanguinius' sons are attacked by previously unknown daemon forces from the warp in an attempt to remove them from the greater battle soon to happen elsewhere. This one was alright. It's always good fun to read about a legion without much background in the story thus far. Not incredibly memorable, though.
Shadows of Treachery - a collection of novellas and short stories mostly about the Imperial Fists and Night Lords legions. Very entertaining, especially the ones featuring Curze.
Angel Exterminatus - Fulgrim leads Perturabo into the Eye of Terror, attempting to kill him as an offering to Slaanesh. Perturabo escapes, but Fulgrim is still able to attain his daemon form, and the Emperor's Children vanish. The Iron Warriors go deeper into the warp maelstrom. I really enjoyed this one. It was good to learn something about Perturabo and his legion, and to see Fulgrim reach his apotheosis.
Betrayer - Lorgar and Angron rampage across Ultramar feeding the warp storms that isolate it and the Ultramarines from the greater heresy. I'm only about halfway through this one, but am really, really liking it. I had no idea the World Eaters could be fleshed out so compellingly, and it's so good to have Argel Tal and Lorgar back after the events of The First Heretic.
Flatland: A Romance of Many Dimensions - NOT a Horus Heresy novel, though some of the worlds it describes do seem pretty dystopic. It's an interesting blend of 1880's Britain social satire and geometric thought experiments. It was pretty short, and a nice palate cleanser between a couple of the aforementioned romps.