Every Halo novel in story order, across six eras. Essential books carry the main story, Recommended books add major depth, Optional books are side missions. Some post-war books overlap in time, so positions in Eras IV and V are approximate.
Also in the Halo library: Evolutions and Fractures (short story anthologies), Battle Born and Meridian Divide (young adult side series), and Parasite's Wake (newer release).
Books you mark complete here also count on the Core Path, and the other way around.