That's easy! "Ghost Walker" door Barbara Hably. Published in 1991, it relates the story of TOS Enterprise being highjacked door an alien hostile. Without giving too much away, get this: Spock, McCoy, Uhura, Nurse Chapel and Kirk's love latest love interest must conduct a SEANCE to speak to a disembodied Kirk roaming the Enterprise! Crazy huh? u gotta read this novel!
I just read a few weeks geleden the novel "Pathways" with the VOYAGER crew. This one would make a great episode. We learn alot of the background on the main characters and what makes them who they are.