"Mission to Horatius": until "Star Trek: The Motion Picture", it was the first Trek adventure in which both Janice Rand and Pavel Chekov appear.
I had a weird/joyful experience with "Mission to Horatius" (1968). One Christmas, my grandmother gave my brother and I a "Whitman Authorized TV Editions" book each one Christmas. I received "I Spy" ("Message from Moscow" by Brandon Keith), based on a show my Dad really loved on TV, and my younger brother received "Star Trek", based on a show we knew about, but did not watch as a family. I had a stint in bed, confined with a bad flu bug, and I finally read both books (and many other unread books on our shelves). I quite enjoyed "Mission to Horatius", but the rat story reminded me too much of "The Trouble With Tribbles" when I eventually saw that episode in 1975.
The Whitman books eventually moved to "the toy box" in my brother's room. After our teens, my brother moved out and I discovered Trek via "The Motion Picture" in December 1979. A few weeks later, I was rummaging second hand stores for Bantam and Ballantine Trek tie-ins, and found a bagged "Mission to Horatius" for about $AU 70.00 - a huge amount, especially in those days. I couldn't get home quick enough. A quick rummage through "the toy box" of stuff my brother had left behind... and there it was. Such a thrill, even though I knew the store had overinflated their price.
Eventually, Pocket Books would release an authorised "facsimile edition" reprint (1996):
More recently, I decided I needed the Whitman "Green Hornet" title ("The Case of the Disappearing Doctor", also by Brandon Keith) for my "Batman '66" collection as well, resulting in several weeks of frantic eBay patrolling. (I think I also have a copy of "Mission Impossible: The Money Explosion" by Talmage Powell, in storage. It was sent to me by a friend, who had noticed that Leonard Nimoy appeared on the cover as his character, Paris.