Contact Harvest isn't extremely relevant to the story but it is at the same time. That book details Johnson's earlier life in the Core but not about the ORION Project. I believe that gets detailed in Halo: The Flood and Ghosts of Onyx. Ghosts of Onyx goes into detail more about the Spartans and more specifically the creation of the S-III's. First Strike is primarily about the Master Chief and the other S-II's if I remember right but its been years since I read the 1st books. I read them years ago, before the 2010 REPRINTS of the books came out. You can find tons of info on Halopedia or the Halo wiki. I prefer to use HALOPEDIA but I always try to read the novels 1st. You can find out which books to read chronologically just by googling it, and if you look up plot synopsis' of the books, I'm sure you can figure out which ones detail the Spartan's the most. What I know is mostly lore, however those websites offer trivia facts, dates, locations, etc etc. Look into those and they'll pretty much detail everything in the universe. The OP [i][u] Lord of Admirals[/u][/i] is a huge Halo Bible of knowledge as well, so at some point he'll probably comment and correct me here and there & point you to the right info.