(written from a Production point of view)
The Lost Years is a four-book Pocket TOS series, published by Pocket Books between 1989 and 1995. It tells the stories of the Star Trek: The Original Series characters in the years between the end of the five-year mission and the events of Star Trek: The Motion Picture. The concept was derived by Robert Greenberger, and the series was originally intended as a trilogy, starting with a "Giant" novel by J.M. Dillard.
The War Virus
The War Virus was originally announced as the third book in the trilogy, alongside The Lost Years and A Flag Full of Stars (The Lost Years, Acknowledgments). The novel was to be written by Irene Kress. It was never released, with later Pocket Books editor John Ordover noting that the novel "didn't work out for reasons I can't really get into".