> Is the TeXBook, chap. G now obsolete?

No, app G explains how the math typesetting uses font parameters from
the tfm files, but a tfm may have other font parameters that are
notautomatically used by tex but may be extracted by the \fontdimen

Th ec fonts (probably only these) have the extra parameters as
described. TeX, itself doesn't use them but you may use the values
in your tex code.