Space-Time Structure by Erwin Schrödinger (1950) [pdf]

This seems to be a very enjoyable read. Even if you just skim the equations, it has a really nice perspective on building up relativity and some alternative pathways.

Funny enough, I had just gotten to the chapter on Schrödinger's derivation of his quantum equation in "The Age of Entanglement" by Louisa Gilder, a book whose style of historical scientific fiction reminds me of reading Asimov's Foundation series.