Greg Egan has a great ability of building fictional universes that are utterly different from ours, but which feel as natural as our day-to-day life. This story is set in a universe in which the speed of light depends on its frequency, and in which time is a dimension that is much more "navigable" than in ours - in a sense, time and space are much more similar to each other in that universe. The beings involved have a biology that is about as different from ours as possible, while still going through much of what we go through, and we can follow as their scientists do the equivalent in their universe of discovering special and general relativity. There are lots and lots of diagrams throughout the book describing how physics works there (and also an appendix with more details, plus about 80,000 words of explanations in the author's website - no, I haven't read everything yet)... and it's about as mind-bending as relativity, to be honest.

Oh, it also has a very interesting plot, as well. Being the first book of a trilogy, I really hope the next ones will show up soon.