@TKB12345678, that's not as clean an answer as you might think. "Becoming Determinant" is first chronologically, but it covers "Divergent" and the first part of "Insurgent" from Tobias' point of view, so as long as you've read those books, you can start with "Determinant" if you prefer. "Prior Rings" overlaps "Determinant" for some time periods, but between the two of them, you should definitely read "Determinant" first. So, basically, it's up to you whether you start with "Becoming Determinant" or "Determinant." :-)