Yea, 12 books. One of them is a collection of short stories, Evolutions, but it still counts. Ah what the heck, here's the list in chronological order:
Fall of Reach, The Flood, First Strike, Ghosts of Onyx, Contact Harvest, The Cole Protocol, Evolutions, Cryptum, Glasslands, Primordium, The Thursday War, and Silentium.