ON-X (and a bit later, Jimmy T) pointed out in the HBO Forum that the PC Halo CD contains a PDF file with a preview of the upcoming Halo novel, Halo: First Strike. This book is supposed to cover the period of time between Halo and Halo 2.

In a related item, Eric Trautmann (MS' novelization guy) has a blog, and the entry for October 3 cites that the galley proof for Halo: First Strike is finished, paving the way for the novel to be released pretty soon.