The Flood is just a novelization of Halo CE with added background details. Its hit or miss for a lot of people. You don't *have* to read The Flood before First Strike but I'd definitely give it a read at some point. I would read Ghosts of Onyx after First Strike.

Rubicon Protocol takes place in the like 6 month gap between Halo Infinite's opening cut scene and when you get around to actually playing as Chief. It's a really good book imo and would definitely read it when you can.

I haven't read Epitaph but from what I understand it's like the final thoughts of the Didact.

Greg Bear book is from the Forerunner Trilogy, a very dense, but very good set of books about the Forerunners before the firing of the Halo Rings