Way back in my first year of university one of my computer science tutors, I think it was Chris Bullivant, gave me the somewhat puzzling advice never to mix Lebesgue integration and black magic. (This was before he evicted me and Catherine Playoust from his tutorial because our discussion of Knuth's Fundamental Algorithms was distracting him from teaching the rest of the class what a for loop was.) I'm wondering now if this remark was inspired by Kennan T. Smith's A Primer of Modern Analysis, which as well as explaining Lebesgue integration carries the (unexplained) subtitle Directions for Knowing All Dark Things, Rhind Papyrus, 1800 B.C..
The Rhind Papyrus is one of the key documents for our understanding of Egyptian mathematics (on which I highly recommend David Reimer's Count Like an Egyptian) but the connection between mathematics and "dark things" continues into much more recent times. Isaac Newton was seriously involved in occult studies, to give just one notable example.
Anyway, having spent some time with geometry and combinatorics, my next entertainment is going to be the Smith book, after which I plan to read Neuenschwander's Emmy Noether's Wonderful Theorem and Manton and Mee's The Physical World.