We have a brand new novel available now that takes the story forward after the events Halo 5: Guardians.
The name of the novel is Halo: Bad Blood and written by Matt Forbeck a writer and a game designer himself.
The narrative follows the Spartans from Blue Team and Fireteam Osiris as they struggle to remain one step ahead of the rampant artificial intelligence Cortana. Spartan Edward Buck is the focal point of the narrative, taking on a secret mission to help reclaim some ground after Cortana’s devious advances.
He reforms his old team, Alpha-Nine, to complete said mission, and first directs them to head to Sanghelios in an effort to meet with any UNSC personnel who managed to survive Cortana’s attacks. It’s a tumultuous journey from there, of course.