Until recently, it was believed to be the James Blish Star Trek Readers (short story versions of the episodes of the original series), and the 1970 novel Spock Must Die!, also by James Blish. But it was recently discovered another novel, Mission to Horatius by Mack Reynolds, was actually the first Star Trek novel, published in 1968 for children.
http://en.wikipedia.org/wiki/List_of_Star_Trek_novels