Whether Walter Matt Jefferies shouted "veto" when he heard about this idea of a "Constitution Class" is unknown, but it's obvious that by 1968 the creators / producers hadn't settled for a "Constitution Class", otherwise the book wouldn't refer (twice!) to an "Enterprise Class".
Well, my hunch is that it's not so much that the creators/producers hadn't settled for a "Constitution Class." I think it's more likely that they had simply forgotten that they had actually already done so back with the terse comment in "Space Seed." I guess all we really know was, at the point that TMOST was written, Whitfield was unaware. Certainly, Bjo Trimble knew by the time her first version of the Star Trek Concordance was published in 1968 that the Enterprise was a Constitution Class.
http://img.photobucket.com/albums/v219/CaptApril/FirstReferenceToConstitutionClass.jpg
The interesting thing is that the Writer's Guide, 3rd edition, dated April 17, 1967, only has this to say about the class:
Now, I presume that WGIII is a reference to this document (Writer's Guide, 3rd edition) since the majority of the description is lifted verbatim but the word Constitution does not appear at all in this edition of the guide. Don't know the reason for the substitution but the influence by the Concordance over early fandom's perceptions of the show cannot be denied.The U.S.S. Enterprise is a spaceship, official
designation "starship class"; somewhat larger
than a present-day naval cruiser, it ...