Incandescence by Greg Egan

A good story in the "strange alien society" type of science fiction. By far the most convoluted physics of any of his books, although that might just be my inability to mentally visualize the world of these aliens - if you follow the link, you will see that Egan has both an easy guide to how it works, and also a non-easy one with all the mathematics. I found the easy version kind of gave me a bit of an idea how it works, but not enough to really get it all. The story of the non-industrial aliens discovering general relativity is compelling, and the parallel story of a post-human and his companion who are searching for the world is good too. I liked it, but you'll need to be prepared to either seriously engage with the physics or skim past those bits to enjoy it.