The font is "Computer Modern," the default LaTeX font. Nothing screams "I'm an academic engineering textbook!" quite like Computer Modern.

Concrete Roman could compete: http://www.tex.ac.uk/FAQ-concrete.html

It is amazing that Knuth still had time left for some mathematics, between designing fonts :)

(Obligatory: http://www.xkcd.com/974/ -- but Knuth solved the arbitrary condiments problem, and others besides.)

The Concrete Mathematics book that resulted was a masterpiece in my opinion. I learned so much about how to computationally do discrete math from that book. And it's a very elegant package.

It's amazing to me that he designed a program (METAFONT) to design fonts for him, and that the various outputs of this program actually are pretty good fonts.

Didn't Hermann Zapf design the math font to go with CCR?

[Answer] Yep, Euler: https://en.wikipedia.org/wiki/AMS_Euler

Here's a handy service for that: https://www.myfonts.com/WhatTheFont/

Just submit a screenshot image of some glyphs of the font at a reasonable size, and let the service figure out the font!

Pretty handy service. Any idea what's the mechanism behind the recognition software?

The default TeX font: https://en.wikipedia.org/wiki/Computer_Modern