Chief finds his fellow Spartan, Linda, in a cryogenic pod that was jettisoned into space before the Autumn made a hard landing on Halo. They rendezvous with Johnson et. al. Then they take a Covenant cruiser and go back to Reach, rescuing Halsey et. al. They learn of a massive Covenant armada on its way to Earth and set out to blow it up with a NOVA bomb (planet destroyer.) They manage to partially succeed. Go back to Earth, opening of Halo 2 happens soon after. Those are the basics, there's obviously more detail. Been a while since I read the books too, gonna go through them again while playing the MCC.