I have read a lot of Star Trek novels and fan fiction over the years, but this book remains my favorite. It's almost exclusively a Kirk-Spock book for long stretches (exploring their unique friendship), although it contains my all-time favorite ST villain in Omne, a rogue leader of an anti-Federation insurgency of bad-ass planets.
The backcover teaser:
Captain Kirk is dead--long live Captain Kirk!
Spock, Doctor McCoy and the other crewmen of the Starship Enterprise experience a stunning double shock. The first, painful blow is Captain Kirk's tragic death. Then, Captain Kirk's miraculous rebirth reveals the most awesome force the Enterprise has ever encountered. Spock is forced into a desperate gamble for Kirk's human soul against Omne--the ultrahuman emperor of life beyond life, and death beyond hell . . .!
Bonus: The " Romulan commander" (Joanne Linville) is brought back from "The Enterprise Incident" episode (season 3) and plays a big role.
The sequel to the novel (The Fate of the Phoenix") is almost as good.
Both were written by Sondra Marshak and Myrna Culbreath.