It's been mentioned before a couple years ago, but I highly recommend The Steerswoman's Road. Definitely what we would consider to be rationalist "fantasy", though as the protagonist applies logic and scientific reasoning, it becomes more of a sci fi.

From a blogger who explains better than I could: "Too much science fiction glorifies mere scientific fact and appeals to scientific authority. Such books are doomed to obsolescence as the state of the art passes them by. Rosemary Kirstein’s books, in contrast, are made timeless by their emphasis on the process of science, which anyone can do. The Steerswoman is a fun work of fantasy fiction with dragons, sword fights, and magic — and also a well-honed work of science fiction, demanding to know the answers to hard questions and the logic behind the magic. The Steerswoman lets the reader watch as the characters use the scientific method to discover the true nature of their world. I cannot recommend this book, and its sequels, highly enough."