Galaxy in Flames.
the first betrayal by Horus begins with the annihilation of the loyalists within the traitor legions, i.e. Lunar Wolves, Emp Children, World Eaters. It is the conclusion, I think, of the Lunar Wolves story (their point of view in the heresy), involving Loken, Torgaddon, and the Mournival. Some new faces are introduced...Garro for instance. I wonder if we will see some different Legions in the next book...
The Horus Heresy artbooks are pretty sweet. There are a few dodgy pics, but on the whole, the standard is great. Some of the best pics are by Paul Dainton, Alex Boyd, Karl Kopinsky, Adrian Smith, Sam Wood, and John Blanche.
The most recent edition has some outstanding Primarch pieces by Alex Boyd.