Digital Typography, Donald E. Knuth, 1999 (Amazon)
This collection of more than thirty articles and notes covering the Knuth’s foray into digital type in the late seventies and eighties. They range from font design (a whole chapter on the shape of the letter S), to the history of typography, to some TeX related material (the entire exposition of TeX’s line-breaking algorithm, for instance). There is also plently of the concrete mathematical analysis associated with Knuth, including a piece explaining why arrowheads plotted on bitmap displays can look asymmetrical even at quite high resolutions.
Perhaps the most appealing of the less-technical articles is Chapter 17 (AMS Euler), which describes the collaboration between Knuth and Herman Zapf to produce a new maths font for the American Mathematical Society. It’s drawn directly from the correspondence between the two as the maths of MetaFont progresses along with the design of the font itself.
The book is, as you would expect, beautifully typeset and bound. It costs about sixteen pounds.