Mathematics by David Bergamini, part of the Life Science Library.
I read some portions of it when I was in elementary school, and some later, and other parts I still haven't read. From that book I found out that such a thing as mathematics exists.
On page 191 we see the back of a boy's head. The accompanying caption on page 190 says "As the youngster at right shows, most human heads have a fixed point, in the form of a whorl, from which all the hair radiates. Topologically, it would be impossible to cover a sphere with hair---or with radiating lines---without at least one such fixed point. For the same reason, the wind cannot blow everywhere over the earth's surface at once: there must be a point of calm." So it was asserting that that could be proved simply by abstract reasoning. When I was in elementary school, I found that amazing.
Then in 9th grade, I read C. Stanley Ogilvy's Excursions in Geometry. You'd be very callous if you didn't find that book beautiful.