In this book (and the series that follow it) we learn about events leading up to and during the Horus Heresy. A period when half the forces of the Imperium rose up in rebellion against the Emperor lead by Warmaster Horus, the Emperors most trusted Primarch.
I can with no word of a lie say, Horus Rising is one of the finest pieces of fantasy fiction I have ever read (and I have read plenty)! What makes this book such a pleasure to read are the characters, they feel like real people dealing with real issues.
If you have in the past been put of buying Black Library novels because of previous poorly written books (The Blood Angels ones spring to mind), then do yourself a favour and go out and buy this one, it is a treat to read and have already started reading it again.