I was wondering if people had recommendations for mathematical essays (by this I mean essays on a mathematical topic, not necessarily essays written by mathematicians).

A person who I used to find entertaining to read at high school was Isaac Asimov, although for some reason he did not write many essays on pure mathematics. In particular I remember the essay collection X Stands for Unknown, which had several entertaining mathematics essays, most of them with a combinatorial flavour if I remember rightly.