What are some examples of "visually beautiful" math texts?

Here's a very old suggestion: Euclid's Elements. Specifically, Oliver Byrne's 1847 rendition of the text with its elegant diagrams and design aesthetic, and Nicholas Rougeux's online conversion of the text into a digital edition, available for free here.

• A Topological Picturebook by George K. Francis

  is full of enlightening pictures of topological structures. Many of them are little masterpieces of drawing art and support this way a better understanding of the theme.

Sometimes it might also be interesting to see behind the curtain.

In A mathematical typography a grandmaster shares with us some ideas about this subject. Here we can get a glimpse about the extraordinary creativity and the deep knowledge which is necessary to produce aesthetically pleasing mathematical texts and pictures.

The book of Douglas Hofstadter, ''Gödel, Escher, Bach - an eternal golden braid'', is an excellent example how lively mathematics can be. The pics of Escher are impressive.