My first Star Trek book, probably Christmas 1971. I was a big fan of the Whitman TV tie-in books; I had The Invaders, Mission: Impossible, The Rat Patrol, Hawaii Five-O, one of the Man From UNCLE books, Bewitched (that might technically have been my sister's)... I've still got the Invaders and Bewitched books, as well as Mission to Horatius, of course, and for the hell of it I got a Voyage to the Bottom of the Sea one recently.
It definitely has to be read as a product of its time. Reynolds wrote it while the show was still on the air, so if he watched any episodes, it would have been maybe the first season and possibly a little of the second. And he was an old school SF writer, not one of the hip young guys like Ellison and Spinrad. But I liked it well enough, and I was glad that Pocket brought it back into print in the '90s.