Of course the other way to handle reduce the hyphenation would be to use left justification instead of full justification (this will cause a ragged right edge). ie add 'text-align: left;' to an appropriate set of selectors. This will however require you to turn 'Publisher Defaults' on for the book on your NC, since the BN custom style sheet adds in 'text-align: justify', and has a higher priority than the document css settings.
(EDIT: I notice that this is what Jerseyman first suggested (post 6)
I just personally find it distracting and wasn't happy with [it]...
Hmmm different strokes for different folks, I guess. When dealing with full justification, you are the first person I've heard of that prefers playing with the interword/intercharacter spacing over hyphenation. (In c r eas e d spa c in g dri ves me batty