According to danhausser's site, "Mission To Horatius" is considered a Bantam book...
Huh? Well, he could hardly put it in a timeline of one book. It's a Whitman book (and the celebratory reprint a Pocket book), definitely not a Bantam, so at best it fits with Danhauser's statement that it is one of "the classic STAR TREK novels" which "were only seventeen in number".
I have the Phoenix novels too, so I hope I like them better than some people.
I read 'em in reverse order, and it didn't seem to matter, nor that I'd not yet seen "The Enterprise Incident".
Remember the scene in the episode where the Romulan Commander swings around in her chair to reveal she is... a female? Not long after ST II came out, a Saavik-besotted friend was desperate to see "The Enterprise Incident" and I finally had it on video, so she watched it, knowing that rampant fan rumors of the day abounded that the Commander and Spock were (possibly) Saavik's parents. And then the Commander swung around in her captain's chair, just as Saavik had done at the beginning of ST II.