Alright, thanks :)Well...not exactly. Horus Heresy refers not only to Horus betraying the Empire, but also the whole galactic civil war that he ignites. The first books are pretty much how things get started.
The usual approach to Horus Heresy books is to read the first ones and then just pick whatever seems interesting. There are some illustrations to be found by googling that show how the chronology and how the myriad plot lines connect and those can be used a rough guide.
My advice is to read the first three, then The Flight of the Eisenstein and Fulgrim. At that point look for books that a) are by Demski-Bowden or Abnett b) are about a legion or an event that interest you.
I think I'll finish the first 3 books and then start reading some 40k stuff, get back to HH a bit later.