"Horus Rising" through "Heralds of the Siege" What can I come back to later, what is essential?

Do I really need to read 52 books to understand "Heralds of the Siege?" I just want the main Horus story line and I'd like to see what happens to Loken. I've read the first two and am just overwhelmed by the mountain of reading in front me, so any help is appreciated!