I'm not exactly clear on the reasons behind his hatred. It was a tie-in technical book... why would he even care enough about it to actually hate it?![]()
Probably, I assume, because Doohan would be challenged by certain fans at conventions, in both public forums and in 1:1 signing experiences, to "explain" aspects of the book that they either perceived to be inaccurate, scientifically impossible or at odds with "canon". There was also the assumption that he'd read the book; his character's name is on the cover, so surely "Scotty" would know!

Certainly the same thing used to happen to Gene Roddenberry, which is why Richard Arnold started to attempt to explain what "Star Trek canon" was! And probably regrets opening that hornets' nest. Fans who'd read the licensed novels, comics, technical manuals and RPG materials would corner GR (and RA) to demand to know (in letters and convention questions) why the Rihannsu weren't acknowledged in the movies, why onscreen TMP background info (and the novel "Dreadnought") info mentioned dreadnoughts if GR was so opposed to them, endless debates about warp speeds, stardates, Klingon forehead crests, and intersellar distances, etc, etc, etc.
Simply saying "No comment", or explaining that GR had little or no control over licensed tie-ins (until 1986, and even more in 1989-1991), would not stem the endless barrage of questions.