Completely false, long-discredited rumors. The claim was that Alan Dean Foster had ghostwritten the TMP novelization, and this came about for two reasons: One, confusion with the Star Wars novelization from two years earlier, which Foster had ghostwritten under George Lucas's name; and two, an overseas translation (French, I think) that mistakenly lopped off the latter 2/3 of the writing credits (screen story by Alan Dean Foster, screenplay by Harold Livingston, novelization by Gene Roddenberry) and thus gave the impression that Foster was the author of the novel.
Just located my copy of the 1980 French edition (J'ai Lu, 1980, seems to be the first French edition), and the relevant page reads...
GENE RODDENBERRY (at top of page, followed by big gap)
un roman tire du scenario de Harold Livingston
sur une idee de Alan Dean Foster
traduit de l'americain par Herbert Drai
(roughly: a novel taken from the screenplay by Harold Linvingston based on an idea by Alan Dean Foster, translated from the American edition by Herbert Drai).
Can't see how that could be the origin of the ghost-written myth, unless someone who didn't speak French had just guessed at the meaning and latched onto the lack of any separate mention for Roddenberry at the bottom. Could be a later edition, or another language, of course.