inter-word spacing with fontdimen

[shmibs]

it feels like there are a lot of hyphenated word-breaks, and that slightly adjusting the fontdimen values might make the book more smoothly readable without increasing page count. here is a comparison of the current print edition (seems to have fontdimen3 of 1.25pt and fontdimen4 of .83333pt) on the left and a version with
\fontdimen3\font=1.8pt\fontdimen4\font=.84pt
on the right.

maybe that isn't ideal either, but it could be played with a bit.