The Forerunner Trilogy in particular is considered "essential lore," kinda like the Horus Heresy for Halo

I got through ghosts of onyx and completely lost the plot (the fall of reach was peak though.)

I have Fall of Reach, Shadows of Reach, The Cole Protocol, Silent Storm, Bad Blood, and (I think) The Flood. Those are all pretty good. Cole Protocol is Captain Keys, Silent Storm is Master Chief lore, Bad Blood picks up right after Halo 5 from Buck's perspective, The Flood is a retelling of Combat Evolved, and Shadows of Reach is more Master Chief lore.

I remember the flood and shadows of reach, I just stopped after shadows of onyx.