Reddit mentions of Excursions in Number Theory (Dover Books on Mathematics)

Excursions in Number Theory by Ogilvy and Anderson manages to touch on a lot of the most beautiful and interesting results of elementary number theory with almost no prerequisite knowledge. Number theory is a really concrete and easily visualized field of study and doesn't rely on a lot of abstraction like some other fields do. The proofs are also in general pretty simple but illuminating. It was my first math book. Definitely 10/10 recommend.

Edit: I think there are a few others in the Excursions series too, which makes reading about several fields simultaneously pretty easy, what with consistent notation/style and all.

Depends what kind of math you're interested in. If you're looking for an introduction to higher (college) math, then How to Prove It is probably your best bet. It generally goes over how proofs work, different ways of proving stuff, and then some.

If you already know about proofs (i.e. you are comfortable with at least direct proofs, induction, and contradiction) then the world is kind of your oyster. Almost anything you pick up is at least accessible. I don't really know what to recommend in this case since it's highly dependent on what you like.

If you don't really know the basics about proofs and don't care enough to yet, then anything by Dover is around your speed. My favorites are Excursions in Number Theory and Excursions in Geometry. Those two books use pretty simple high school math to give a relatively broad look at each of those fields (both are very interesting, but the number theory one is much easier to understand).

If you're looking for high school math, then /u/ben1996123 is probably right that /r/learnmath is best for that.

If you want more specific suggestions, tell me what you have enjoyed learning about the most and I'd be happy to oblige.

I like reading math books for fun, especially cheap Dover books. Excursions in Number Theory by Ogilvy & Anderson (lots of cool little stuff). Introductory Graph Theory by Chartrand (a lot of real-world programming boils down to graph theory). An Introduction to Algebraic Structures by Landin (abstract algebra). In Code: A Mathematical Journey by Flannery (modular arithmetic, factoring, and cryptography). In Code or Excursions would probably help with Project Euler 3 and several others.