Spock's first name is given as "S'chn T'gai" in the 1985 novel Star Trek: Ishmael by Barbara Hambly.
Spock's last name is given as "Xtmprsqzntwlfd" in the 1980 book U.S.S. Enterprise Officer's Manual by Geoffrey Mandel and Doug Drexler. It was DC Fontana who came up with it though, probably as a joke.
Taken verbatim from the
Memory Beta Spock Talk Page:
(I hope
Therin of Andor doesn't mind me quoting his Memory Beta post.)
What name is official and/or canon? None. On screen it's "Spock, son of Sarek".