Keeper of the Lost Cities has a pretty straightforward main series order, but some people forget about the extra content. The core sequence starts with 'Keeper of the Lost Cities', then 'Exile', 'Everblaze', 'Neverseen', 'Lodestar', 'Nightfall', 'Flashback', 'Legacy', and 'Stellarlune'. After that, you've got 'Unlocked', which is a hybrid guidebook/novelette, and the latest release is book 9.5, 'The Graphic Novel Part One', which adapts the first book.
It's also worth tracking down the bonus short stories from special editions, like 'The Secret of the Moonlark' or 'The Forgotten Secret', as they add little character moments. Just don't jump into 'Unlocked' before 'Legacy'—the timeline matters.