Angel Isle Book

Quite different from "The Ropemaker" in spite of a lot of shared background and characters. In fact, they're so different that I find it hard to figure out whether I like one better than the other. This is extremely intelligent, possibly even genius fantasy, and I have to pay really careful attention to even begin to understand what's going on at times. Normally I might find that repellent, but in this case I find it fascinating. Magic as a mathematical concept is not normally the way to my heart, but this is such a high-level mathematical concept that I could never in a billion years understand it as math, so it's really just as magical as normal fantasy magic, but somehow much cooler, because the fact that it's apparently based on concepts that exist in our world make it seem (to me anyway) more likely that really exciting magic, too, can exist in our world.
Outside of the genius-level concept of magic, a lot of really cool stuff happens, and as with "The Ropemaker", the people are really good: complex, confused, interesting, strange, kind, grumpy and very real in so many ways.