Edge fluting is our optical/perceptual response to closely valued abutting color fields. This is a common effect in gray scales, one of which you've seen on page 92 of Envisioning Information.
Rather than seeing each field as a flat or fixed color, the color interaction at the edges of the abutting fields induces in us a sensation of a new "color." This new color is localized at the edges of the fields and the resulting look is that of curved or recessed panels. Think of the fluted columns of a Doric temple (hence the name), or, more prosaically, closed mini-blinds. The effect can also be seen, in a less pronounced way, at the edges of a single color field and the surrounding color, say a white rectangle on a gray page.
Back to page 92, note how each gray panel has a darker left edge and a lighter right edge, while the "native" color of the field remains in the center, resulting in a "scooped" look. That's edge fluting.
-- Steve Sprague (email)