I also have
Mission to Horatius, but it's the 1999 hardcover reprint. The original was released in 1968. The copy of
Spock Must Die! that I have might be an original, it's copyright 1970. (I got it at a used book store) I don't know fast they whipped out these books back then, so it very well could be the 2nd one. I know that Star Trek books didn't really pick up steam until the 1980s, when they started coming out with a lot of them a year, so it could have been 2 years between books back in the 60s and 70s.