At first, I thought the font wasn't designed by Erik, since it didn't appear to match any of his output.

Then, I discovered this: http://usshop.gestalten.com/hello-erik.html

The font is called Real, and was designed by Spiekermann specifically for this project. You get it free when you buy the book.

Good reasoning... good sleuthing...

ah! makes sense, thanks!

Real is Spiekermann's redesign of beloved Akzidenz Grotesk in one weight only especially for this book. Very readable.