I was under the strong impression that Roddenberry had written two.
The other Roddenberry novel was to have been "The God-Thing", based on an earlier premise for what might have become TMP, the idea was shunted aside - and pilfered for some of the story points in TMP. (Plus, Shatner's ST V would later also mine the "Enterprise searches for God" aspect.)
The fan club I belonged in the 80s and 90s used to receive regular updates on Gene - and Susan Sackett would often report, between TMP and TNG, that GR was "off in his home office, busily turning 'The God-Thing' into a novel". Several other authors supposedly had a go at helping him with it, at various stages, including Walter Koenig.
After GR's death, Pocket approached Majel Barrett with the idea of Michael Jan Friedman expanding the numerous fragments GR had written, and completing the novel. The book catalogues even carried display ads for both the hardcover and the audio production! However, Majel was disappointed with the result - she felt as if MJF had added too much new material and brand new characters, IIRC - and the novel was never published.
At a convention here in Australia in the mid 90s, Majel told me (via an audience question asked by me) that she had turned over the original work to David Alexander, the guy who did GR's authorized biography. Alexander was a former editor of "The Humanist" magazine and had never written a novel before. Alexander had no contract with Pocket Books, no agreements with the editor, and Majel knew that there was no guarantee that Pocket would take it on - and that no other publisher would be able to, due to licensing - and that's probably where the story ends.
Steve Roby dedicates a whole webpage to the saga:
http://www.well.com/user/sjroby/godthing.html