By the power of truth, I, while living, have conquered the universe.
Given that Primordium is less than a month away from release, you should read Cryptum first so you know what's going on.
Then read The Fall of Reach, The Flood, First Strike, Ghosts of Onyx and Glasslands because they're all directly linked.
Then move on to the stand-alones - Contact Harvest, Cole Protocol and Evolutions.