[Jtsummers]

Mathematical Games, by Martin Gardner.

EDIT: Distractions abound so I hit submit before forgetting.

Why: Martin Gardner wrote on a variety of mathematical topics in fields such as geometry, graph theory, number theory, combinatorics, topology, and beyond. His writing is very approachable, and well sourced. This will help to develop a base vocabulary across the mathematical fields that you can use for further research and investigation of your own once you find the areas that interest you, along with being delightful reads just for their own sake for the mentally curious and engaged.