Star Trek: The Lost Years was a series of TOS novels set in the period between the end of the TV series' run, and Star Trek: The Motion Picture. The first book, The Lost Years by J.M. Dillard, was published in 1989, and in that book's acknowledgments, the titles of the two other books in the planned trilogy were given: A Flag Full of Stars and The War Virus.
A Flag Full of Stars by Brad Ferguson was published in 1991 after being delayed and heavily rewritten by Dillard. The War Virus by Irene Kress was ultimately canceled, and two other novels, Traitor Winds by L.A. Graf (1994) and Recovery by Dillard (1995) finished the series.