Why is Time's Echo book 1.5 rather than the first chapter or first few chapters of Time's Edge?
Time's Echo is a novella from a different character's perspective. You could understand the series without reading the two novellas (there's a second one coming between Time's Edge & book 3) but they give you some additional background on the Cyrists and things that it would be difficult for a reader to discover through Kate's perspective alone. In Time's Echo, we get some backstory on Kiernan and his life with before Kate, and the events that lead up to their meeting in Timebound. It also lays the groundwork for a few things that will be covered in Time's Edge.