Start with high-confidence relativity books, then narrow by subgenre or adjacent topics.
Black Holes
Brian Cox, Jeff Forshaw
Functional Differential Geometry
Gerald Jay Sussman, Jack Wisdom
Why Does E=mc2?
The Order of Time
Carlo Rovelli
Fantastic Numbers and Where to Find Them
Antonio Padilla
The Manga Guide to Relativity
Hideo Nitta, Masafumi Yamamoto
