Genius At Play, Roberts
Biography about the life and work of John Horton Conway.
I thought this was really good! I listened to the audiobook version and it was just maths-y enough to be understandable without actually having a paper copy in front of me (though I did refer to the accompanying PDF a few times).
It was a really nice mix between finding out about John Conway himself, looking at actual concepts like the surreal numbers and groups, and a higher-level commentary on mathematics in general.