I didn't notice any substantial departures from the episodes in any of the stories except "Friday's Child", which treats the character of Eleen rather differently. "The Doomsday Machine" and "The Last Gunfight" have some small changes, as well.
This book is, like its predecessor, fairly enjoyable. Although I've not found the series to be exceptional, it seems that contemporary readers were more impressed: in the introduction, James Blish describes some of his previous work (twenty-seven novels and short story collections, including a Hugo winner). Then:
I note these figures not to brag--well, not entirely, anyhow--but as background for one astonishing fact: I have received more mail about my two previous Star Trek books than I have about all my other work put together.
He had been receiving letters "at an average rate of two a day ever since January 1967." Of note is that "most of [the letter writers] say that they have never read, or seen, any science fiction before Star Trek, or if they have, that they hadn't liked it." To fans looking for more information, he recommends The Making of Star Trek by Stephen E. Whitfield. I'm reading that book, now, and it's fairly interesting (with some caveats–review forthcoming).
Finally, at the end of the introduction, he very casually reveals his next project:
Thanks, too, to those who asked that I write an original Star Trek novel. Both the studio and Bantam agreed, somewhat to my surprise, that this was a good idea, so it's in the works.
The book in question, Spock Must Die!, would be published in February 1970, nearly a year later, and Blish's next volume of adaptations would not be published until July 1971.
I admit that I'm really looking forward to Spock Must Die! giving me a break from these adaptations. All the same, with adaptations of popular episodes like "Mirror, Mirror" and "Amok Time" (and even "The Trouble with Tribbles", if you're in a less serious mood), Star Trek 3 is a nice afternoon's diversion.