Paperback copies of The Flood, Fall of Reach, Contact Harvest, and I think that's it. My copies are older versions of the books (Excluding Contact Harvest). I don't have the newer, re-editied, versions.
Reach = best. Then Flood.
Still haven't bothered to pick up Contact Harvest again. Didn't keep my interest.
EDIT: Oh, and Halsey's journal.
[Edited on 07.23.2011 8:59 AM PDT]