I'll toss in vote N+1 for "How to Solve It" by Pólya: once you get past the hurdle of just understanding notation / language and some of the basic concepts, mathematics becomes much more about problem-solving.
Aside from that...
Oliver Byrne's Euclid: https://www.math.ubc.ca/~cass/euclid/byrne.html - a graphical treatment of Euclid's Elements. Much, much more accessible than earlier renditions, and a great introduction to methods of proof.
Vi Hart's videos: https://www.youtube.com/user/Vihart - she does a wonderful job of conveying the wonder of mathematics in a clear, informative manner.
A Mathematical Mosaic: https://www.amazon.ca/Mathematical-Mosaic-Patterns-Problem-S... - this was one of the books that got me excited about mathematics as a kid. The material is advanced by high school standards, but presented in a way that invites you to think / learn / generalize.