The Halo books have sprawled in a fascinating way, and nailing down a pure release order gets messy because of how the EU expanded. The foundational sequence began in 2001 with Eric Nylund's 'Halo: The Fall of Reach', which is a prequel to the first game. William C. Dietz's 'Halo: The Flood' followed in 2003, a novelization of 'Combat Evolved'. Then Nylund returned with 'Halo: First Strike' in 2003, bridging the first and second games. After that, it splinters. The 'Ghosts of Onyx' era introduced the Spartan-IIIs, and then we got the Kilo-Five trilogy, the Forerunner Saga, and so many standalone entries. A strict list feels like doing a disservice because later series like the Forerunner books by Greg Bear ('Cryptum', 'Primordium', 'Silentium') are best read as a block even though they released years apart from earlier novels. If you're looking for a timeline to read in, publication order is one valid path, but it jumps around the universe's history a lot. Personally, I'd recommend grouping by narrative threads rather than strict release date after the initial five or six books.
Trying to list every single release in order here would be a huge wall of text and honestly, a quick web search for 'Halo novel release order' will give you a cleaner spreadsheet-style list than I can type out. The important nuance is that the early Nylund and Dietz books form the core of the old EU, while everything post-2010 represents a new era of deeper lore integration, sometimes retconning or expanding on earlier ideas. The release of 'Halo: The Fall of Reach' actually created minor continuity snarls with the game that came out after it, which is a fun bit of trivia for lore deep-dives. I'd say grab the first three, see if the military sci-fi tone works for you, then decide if you want to follow the Master Chief thread or branch into the Forerunner's ancient history.