First, it didn't start on Reach, it started on Harvest. Reach was where the remaining Spartan IIs were supposedly lost. I believe the fallen Spartan was either a female or a tranny. With the Spartans, the mission is always first priority.
I found the books a good read and was amazed that these guys could write something so riveting in such a short time period. As an aside, I was glad to see the movie, District 9, not make it to the Halo movie. Although I really like most of Peter Jackson's work, this movie's settings and CGI didn't even come close to laying out the story of Halo, imo.