Oh, yeah, sure, I've always collected those things, and something like that, hitting on so many of my happy-buttons at once (the TREK connection, the Culp connection, the novelization-osity, Weverka as author) I'd've read right off the bat, and did.
Mr. Weverka's novelizations tended to be straight ahead. These were written before studio merchandising departments routinely demanded approvals, so though most novelizations were
relatively faithful, there were authors like Lou Cameron, Manning Lee Stokes, Don Tracy (as Roger Fuller) and a few others who would sometimes put a screenplay through their own personal blender, with mixed results. But Weverka worked with what he was given, and his novelistic reinterpretation tended to be teased and grown out of what was already there, at least as he saw it.
SPECTRE was no different. At a standard paperback "signature" (book page count) of 160, and likely crafted to meet a designated word count, Weverka's SPECTRE novelization didn't have much room to indulge in deep internalization -- only enough to give the thing pace and a point of view, which Weverka did with smooth, if unremarkable efficiency. His style wasn't showy, but he was always an entertaining professional.
The cover never "changed" -- the jpg you see was the debut release. My guess is that the TV movie had, as you suggested, attained enough cult status, owing to Roddenberry's co-authorship of the teleplay, to prompt Bantam to release the novelization, especially as Bantam, at the time, was still reprinting their line of STAR TREK episode adaptations by James Blish, plus a number of originals. (They'd retain the reprint rights to all these but had since lost the franchise to Pocket.)
The following is conjecture, but I'd bet a couple bucks that it's true. I think, despite the above, that it's possible Bantam had commissioned the SPECTRE novelization to coincide with the TV flick and then held back publication when it didn't go to series. 1979, though, was the year the first STAR TREK movie was released, and the revitalization of the franchise in paperback may have prompted them to pull the manuscript out of the drawer and catch some related-interest business.
There was a not-dissimilar delay between Roddenberry's previous unsold pilot, THE QUESTOR TAPES, and its release in paperback by Ballantine Books. And there too there was a refreshed franchise to spur interst. At about that time (1974), Ballantine's science fiction imprint, Del Rey, was publishing the STAR TREK LOG series by Alan Dean Foster, adaptations of the Saturday morning animated episodes. Unlike Bantam Books, though, they did not commission an "outsider" to do the deed, but rather one of the seminal TREK TV writers, D.C. (Dorothy) Fontana. Also unlike the SPECTRE novel, the first printing of THE QUESTOR TAPES contained stills from the film, which starred Mike Farrell and Robert Foxworth in the leads. (Subsequent editions would be published with a non tie-in cover and under the Del Rey imprint.)
