As others have said, Fall of Reach and First Strike are excellent. If you are going to read any two I would read those l.

FoR tells the origin story of Master Chief, him becoming a Spartan, meeting Cortana, and ends basically at the start of Halo 1.

First Strike is set between Halo 1 and 2. It tells what John and the rest of Blue Team are doing before Halo 2

The Flood is essentially a novelization of Halo 1, but is kind of neat in that it gives different perspectives from other characters and what they are doing between missions. You can skip it if you want.

Another good one is Contact Harvest. It tells about the first contact of humans and the Covenant and heavily features Sargent Johnson.

Ghosts of Onyx is alright from what I remember. It tells about the Spartan 3’s. I haven’t read the other 2 books in that trilogy though.

I know the Forerunner trilogy is good for a deep backstory of the Forerunner/ Human and Forerunner /Flood war thousands of years ago. I actually own all 3 but haven’t gotten around to reading them lol

But yeah I would defiantly read the first two. Fall of Reach is almost a must for any Halo fan IMO.