Once I stumbled upon Wooden Books, I had to collect them. These are tiny textbooks on all sorts of esoteric and not-so-esoteric subjects. So far, I have:
The Elements of Music
The Golden Section
The Mayan and other Ancient Calendars
A Small Book of Coincidence in the Solar System
Sun, Moon, & Earth
The Alchemist's Kitchen
I plan to purchase more, but these are very intense to read and sometimes it takes me multiple readings of one page to begin to grasp the concepts. Fascinating!