There have also been audio book exclusives as well that have never seen print....
Transformations: A Captain Sulu Adventure by Dave Stern
Cacophony: A Captain Sulu Adventure by J.J. Molloy
Envoy: A Captain Sulu Adventure by L.A. Graf
Borg by Hilary Bader
Spock vs. Q by Cecelia Fannon
Spock vs. Q: The Sequel by Cecelia Fannon
You might also want to add the audio book adaptation of Fallen Heroes
(DS9 #5) to that list, since while it shares the same cover and story idea, it is written so differently from the novel that it is very hard to call it an adaptation of any sort. In a way it's kind of like how you have two different versions of "The Hardy Boys: What Happened At Midnight" (1931 & 1967), but aside from the story idea being the same, both stories are written and told very differently.