Back when I read it, I came away with the general impression that it was set shortly before TWOK (sort of like Dwellers in the Crucible), but if the glasses are mentioned, then it really has to be an Enterprise-A book...
The Klingon on the cover seems to be vaguely based on Valkris in terms of her facial features, so I never thought much of it (beyond it bolstering the movie-era vibe).
Just did a quick re-read of this one, and surprisingly, there actually are several definite continuity references which might circumstantially point to an
Enterprise-A setting:
1. The post-TWOK glasses references -- in the movie, Kirk and McCoy have clearly never had that particular conversation before, and in this novel, he needles Kirk about it, with the implication that this has now become something of a semi-regular topic. Also, Kirk now evidently has another pair on the ship in his quarters, which would place this story after the events of
The Voyage Home (when he pawned the first pair back in the 20th century).
2. Peter Kirk is mentioned at one point as being significantly older than he was on the TV show, and now a very renowned and accomplished Federation research scientist in his own right. This actually ties into the Wildstorm
Enterprise-A era comic story "Bloodline," set in 2292, when Peter is serving aboard the USS
Feynman as that starship's chief scientist.
3. Close to the end of the book, McCoy drops the old English proverb, "A man at sixteen will prove a child at sixty," on Kirk, in direct reference to his age, and Kirk groans. Now, this is probably a slight rounding-up of sorts on McCoy's part, but if Kirk's sixtieth birthday falls in 2293 and he's now getting very close (as the novel implies), this would still fall squarely into his
Enterprise-A command period (2286-2293).