
1) Horus Rising. This is a tricky one. Pay off happens in later books, but it sets the scene. A secular, aggressive, and humanist Empire in the 30th Millennium. The reconquest of the galaxy is seen as only a matter of time and effort and many of the Sons of Horus worry about what will come after the Great Crusade is over, seeing their place already eroded by Imperial bureaucrats. Horus has been named Warmaster and the Emperor has retired from the front for his own reasons.
It's not a bad book, but it's all set up and the problem is that False Gods is a miss.
Recommendation: Read it only if you want to get a feel for the 30th Millennium, which is does well.
2)False Gods. Horus is wounded by a corrupted weapon and is taken to a place of healing. Unfortunately, the place of healing conceals a Chaos cult and presided over by First Chaplain Erebus of the Word Bearers. Hrous falls to Chaos.
And oh merciful God is the corruption sequence awful. It's really terrible. The scene where Horus gets wounded isn't that impressive either.
Recommendation: Pass.
3) Galaxy in Flames. It's good. Horus uses an uprising on Isstvan as an opportunity to purge loyalists elements before beginning the civil war. Unreliable members of the Emperor's Children, Sons of Horus, and Death Guard are used to attack the rebels and then both forces are bombed from space. Suspect ships are destroyed. Then there is a ground assault to finish off the survivors. The protagonists are, of course, the embattled and doomed loyalists who are being slaughtered by their brothers.
Recommendation: Read. The only weakness is that a lot of the characters are from the earlier books and if you haven't read them then you'll miss out on some character development, but that's a fairly minor weakness.
4) The Flight of The Eisenstein. The famous escape of a single loyalist vessel from Isstvan, bringing warning to the rest of the Imperium of the Heresy. Easier said than done, of course.
Recommendation: Read.
5) Fulgrim. The fall of Fulgrim and the Emperor's Children, after the Primarch takes possession of a daemon weapon and its corrupting influence takes hold.
Recommendation: Pass. It's not badly written, but the descent into depravity is too clean.