here is what library thing has to say about it
http://www.librarything.com/series/Halo
Halo: The Fall of Reach by Eric Nylund Book 1
The Flood (Halo) by William C. Dietz Book 2
Halo: First Strike by Eric S. Nylund Book 3
Halo: Ghosts of Onyx by Eric Nylund Book 4
Halo: Contact Harvest by Joseph Staten Book 5
Halo: The Cole Protocol by Tobias S. Buckell Book 6