Way back in 1969 or 1970, a relative gave me a kids’ Star Trek book. I can’t recall the title at all; however, I do remember some of the key points in the book.
[ul][li]It was (obviously) based on the original series.[/li][li]The Enterprise had to visit three different planets in the same system.[/li][li]One planet made use of doppelgangers.[/li][li]There was a bubonic plague scare on the Enterprise.[/ul][/li]
Anyone know what the title of the book is?
Yes, I do! 
It’s Mission To Horatius by Mack Reynolds. It was recently reissued and is back in print.
OUTSTANDING! Thanks. 
I was hospitalized for six weeks and the nurse who was the night shift supervisor introduced me to the Star Trek TV show. She gave me the book since I got very interested in the show.
Brush with fame: Not only did she share the same family name as the astronaut who didn’t get to land on the Moon for the first lunar landing, she’s his niece.
If you ever need to look up another Star Trek novel, this site might be useful: http://www.well.com/~sjroby/lcars/
I owned a little over 300 of those things at one point. Trek in all its forms was my passion in my teen years.
And it was the first ever Star Trek novel - even before the Blish one.
I got it recently, and it wasn’t very good, but Reynolds was more an idea person than a stylist anyway.
I got it myself when it first came out. I was about 11 and not much of a critic. As you said, it was the only Star Trek novel then and any Star Trek fiction was good.