Gaunts Ghosts - The Founding (omnibus of three books)
Eisenhorn (another three booker)
Horus Heresy - Horus Rising, False Gods, Galaxy in Flames
Space Marine Battles - Helsreach
Ten awesome books there to get you going, all with more novels shooting off from them should you enjoy them.
As an aside, I think everyone should try reading Ravenor, and by extension Eisenhorn(as Ravenor follows on from it) before reading the Emperors Gift, as though minor, it links in to Ravenor, not a huge issue if you don't, but if you do read Ravenor first, what appears to be a rather small and insignificant reveal in The Emperor Gift, becomes a lot more interesting and surprising.