Flatland is a classic, though it's almost a pamphlet. While you asked for a novel, I can't resist recommending the short fiction of Borges.

That's always my problem with these huge lists. I feel like any recommendation is so much more useful if it's accompanied by at least a bit of context about why it's being recommended. Whenever I ask for recommendations on reddit or elsewhere, I prefer 2 or 3 answers with information over one of the 20 item list dumps. This site does actually have some snippets of context, you just have to basically click on one (I guess at random) to get to them. Some categorization or other discovery feature might help greatly I think.

Anyway, I will second the other reply's recommendation of Borges also with the caveat that he didn't write novels. His short stories are phenomenal. They are also a bit more philosophical than "mathematical", but some of them (like The Library of Babylon) deal a bit with the intersection of mathematical ideas (combinatorics and infinity in that one's case) with the way that playing with them would impact people and society.

The best advice for reading him would be, assuming you are reading in English, to pick up Penguin's Collected Fictions edition, containing a good translation of a lot of his short stories (and poetry as well). Skip past the first collection of short stories ("A Universal History of Iniquity") as it's pretty good but not his best. Start with the collection called "Fictions". Its first story is Tlön, Uqbar, Orbis Tertius, which gives you a good taste of how he writes.

I would suggest Uncle Petros and Goldbach's Conjecture - I read it many many years ago, and not only found it very entertaining, it also ultimately sparked my interest in mathematical research :)

if you want a really good book about mathematicians rather than mathematics per se, I can recommended "uncle petros and goldbach's conjecture"

Well, psychohistory is presented as a sort of mathematics for predicting the future, I think it fits the theme.

Likewise, I'm not sure it's on the list, but starship troopers has a kind of mathematics for government (the military dictatorship with citizen-by-merit is provably optimal, not just a random system).

It fictionalized "math" but still fits the generic idea, IMHO.

"many, many more entries" are included. Moby Dick and War and Peace are in here.

Neverness by David Zindell? Not quite 50 years though.

That's where I'd start. If memory serves, Fantasia Mathematica was the first/best one.

And at least one of the recommended works -- The Devil and Simon Flagg -- is anthologize there.

John Nash saw the symbol pi as just a Greek letter. In grad school he proudly used every Greek letter in a paper. I was asked to decorate his (film) dorm window in grease pencil, and by chance a photograph of Russell Crowe through this window became one of the most widely distributed promo stills. He's looking intent behind the equation "0 < pi < 1", straight from Nash's paper.

The reaction in my community, which I quite enjoyed, would disabuse anyone of the idea that mathematicians are less cripplingly conventional than other people. To the majority of mathematicians, pi is tied to the circle. A great UC Berkeley email thread speculated that I was trying to make Russell Crowe look like a fool.

originally attributed to a young Gauss, according to my teacher who loved telling this story.

https://www.nctm.org/Publications/TCM-blog/Blog/The-Story-of...

My favorite:

"Very well. Let's have a love poem, lyrical, pastoral, and expressed in the language of pure mathematics. Tensor algebra mainly, with a little topology and higher calculus, if need be. But with feeling, you understand, and in the cybernetic spirit."

"Love and tensor algebra? Have you taken leave of your senses?" Trurl began, but stopped, for his electronic bard was already declaiming:

Come, let us hasten to a higher plane,

Where dyads tread the fairy fields of Venn,

Their indices bedecked from one to n,

Commingled in an endless Markov chain!

Come, every frustrum longs to be a cone,

And every vector dreams of matrices.

Hark to the gentle gradient of the breeze:

It whispers of a more ergodic zone.

In Riemann, Hilbert or in Banach space

Let superscripts and subscripts go their ways.

Our asymptotes no longer out of phase,

We shall encounter, counting, face to face.

I'll grant thee random access to my heart,

Thou'lt tell me all the constants of thy love;

And so we two shall all love's lemmas prove,

And in our bound partition never part.

For what did Cauchy know, or Christoffel,

Or Fourier, or any Boole or Euler,

Wielding their compasses, their pens and rulers,

Of thy supernal sinusoidal spell?

Cancel me not - for what then shall remain?

Abscissas some mantissas, modules, modes,

A root or two, a torus and a node

The inverse of my verse, a null domain.

Ellipse of bliss, converge, O lips divine!

The product of our scalars is defined!

Cyberiad draws nigh, and the skew mind

Cuts capers like a happy haversine.

I see the eigenvalue in thine eye,

I hear the tender tensor in thy sigh.

Bernoulli would have been content to die,

Had he but known such a^2 cos 2 phi!

   -- Cyberiad. Stanislaw Lem