One outstanding, albeit far from easy, book, is Road to Reality, by Nobel prize winner, Sir Roger Penrose. whilst the motivation is theoretical physics it is really about maths.'Makes you think quite deeply about stuff you think you know - "what is a fraction really?" for example. I wizzed through a fair few chapters thinking how good and accessible it was then it suddenly got harder. Realised that was actually the point I'd got to at university before I lost my way! It's a heavy book, but genuinely is accessible with effort for a numerate lay person

Couple of more popular books about maths but more descriptive than textbooks

A moderately accessible book for someone with some calcuus and the only textbook I kept from university (first year maths). Complex analysis (ie calculus with complex numbers) is actually rather simpler and a lot more elegant to real number calculus like we did for a

level.

A couple more descriptive but still informative books

And another about "the Hilbert problems" - a programme of work to solve the great unsolved problems of the early 20th century as set out by David Hilbert: some have been solved like the Fermat's Last Theorem (by Andre Wiles) and the Poincare conjecture (By Perlman) but many other still open. Semi technical but about the story of the maths rather than a textbook

Another useful semi-popular / semi-technical but still worthwhile book

And on specific subject here are two on Game Theory: first on is partly a biography of Von Neumann with an introduction to the subject and the other is an accessible intro to the subject proper

And I very much struggled with Tensors at uni, and hence with General Relativity for which they are vital. I found this helpful in revisiting the subject 40 years later, though still some way to go

Whilst I did extremely well at A level I rather lost my way at university very nearly dropping out. If you've done the equalivalent of nearly an A level the Penrose should a good book for you; with the more descriptive or specific ones acording to your interests