I have been fairly ripping through Horus Heresy books lately. At this rate 2016 may be the best booklog year yet.
I mentioned Vulkan Lives previously. It was a little lower-key, but did some important groundwork for later books.
I finished that one, then just devoured The Unremembered Empire, which was amazing, featuring four(!) primarchs pretty heavily, and covering some pretty momentous stuff in the Heresy timeframe, Such as the establishment of Imperium Secundus, Curze and the Lion returning to the field, and the apparent death of a second primarch.
From there, Scars rewound the overall timeline a ways to get us caught up on the White Scars and their campaign to far-flung Chondax, where Horus sent them to mop up a greenskin infestation while he did his dirty Isstvan business, kicking off the rebellion. Here we learn who the Scars are, and get an idea of what their primarch, Jaghatai Khan, is like. Space Wolves, Alpha Legion, and Death Guard all also make substantial appearances.
I'm now reading Vengeful Spirit, which is the first book since the opening trilogy to feature Horus himself and his legion as prime actors. I'm only getting started with it, but it looks like another barnstormer, and some of the Emperor's past may be revealed here, as well.