If you want to read something akin to an origin story, you can either read or watch Fall of Reach, which details the Chef's indoctrination into the UNSC.
Nightfall on the other hand shows you what happens Post-Covenant war, but before Halo 5, and details Locke's Pre-Spartan adventures.
If you like the Halo: 4 campaign, you can read the graphic novel Tales from Slipspace to conclude it's story.
Did you like Halo 3: ODST? Great! Halo: New Blood Explores What Buck takes on after the Covenant war, and details how he became a spartan. Halo: Bad Blood will detail Buck's mission for ONI shortly after Halo 5. Its gonna be released this June so be on the lookout.
If you want to explore The Forerunner-Flood war, and consequentially, the Forerunner-Human war, I suggest Halo: Cryptum, Halo: Primordium, and Halo: Silentium
If youre looking for something more stealth like, Id suggest the Halo: Bloodline saga. Good read for Pre-Fall of Reach stories.
Halo: The Flood is essentially a novelization of Halo: CE, with a couple af added anecdotes and side stories.
Halo: First Strike fills in many gaps between Halo CE and Halo 2
The Kilo Five trilogy details the events immediately following the ending of Halo 3, and it's first book Glasslands, Is a sequel to Ghosts of Onyx.
Pretty much all of the Blue Team novels are a must.
Thats really all I can recommend as theyre the ones ive read.