So I stumbled on this series out of order and had to piece it together, which was a whole thing. The core sequence is clearly 'Chrono-Core Awakening', then 'Echoes of the Sundered Realm', followed by 'Veil of the Eternal'. That's the main trilogy. But there's also the prequel novella, 'Whispers Before the Storm', which some people say to read first and others swear you should save until after book two.
Honestly, I read the trilogy first and then the prequel, and I preferred it that way. The mysteries in 'Chrono-Core' hit harder when you don't have the backstory upfront. The prequel answers questions you don't even know you have yet, so it feels more like a rewarding deep dive later on.
After the main three, there's a collection of side stories called 'Fragments in the Spire' that expands on some minor characters. It's not essential for the main plot, but if you're really into the world, it adds nice texture. I'd slot that in last, definitely.