I don't recall whether any thread has gotten into how Roddenberry had received a credit, but then lost it and ended up doing the novel - does anyone here have knowledge?
On the strength of his work adapting TAS episodes for the "Star Trek Logs", Alan Dean Foster was invited to turn a Roddenberry script, "Robot's Return", written for the proposed "Genesis II" TV series, into a Trek story. Roddenberry promised to grant him full "story by..." credit (ie. not take it to arbitration), no matter how much of Roddenberry's original material ended up in the "In Thy Image" story, with Livingston doing the eventual screenplay.
Roddenberry was keen to break into novel writing at the time, hence the novelization. His "The God-Thing" novel suffered numerous false starts and was never completed.