It says on the Pocket Books website that Paramount retains ALL rights to your book, if you sell them one.
Say you have a general concept for an SF movie or book, not involving ST or its characters. Can you keep the rights to the concept and write a movie around it, a non ST one, AND write it as a ST book, with ST characters?
Not sure why you'd want to, but there's some precedent. Larry Niven's Known Space novella "The Soft Weapon" was pretty faithfully adapted by Niven himself into TAS: "The Slaver Weapon." Dennis Bailey and David Bischoff adapted their original novel
Tin Woodman into the TNG episode "Tin Man," although that one was significantly more changed from its original. And David Gerrold has based one Trek novel,
The Galactic Whirlpool, and two original novels,
Yesterday's Children (in two different versions) and
Voyage of the Star Wolf, on the same initial concept which he originally pitched as a TOS episode, though the original novels went in unrecognizably different directions from the initial premise and TGW, so it probably doesn't really count here.
But in all those cases, the original work was published under the author's copyright before it was adapted into a Trek tale. So the author kept the rights to the original work. If you first sold the story as a work of Trek fiction, I doubt you could then rewrite it as an original work. Of course, there'd be nothing to keep you from building a substantially different work on some of the same concepts. For instance, if I wanted to write an original novel about spacegoing life forms, I could do so if it were a distinct story from
Orion's Hounds rather than just the same book with the names changed.
