New Earth
Star Trek: New Earth (continued as Star Trek: Challenger) is a series of Star Trek novels published by Pocket Books in the United States, running for six books as part of a subseries (#89-94) in the Pocket line.
It depicted the events of a second five-year mission taking place between the events of Star Trek: The Motion Picture and Star Trek II: The Wrath of Khan. Some major events shown are the switch to the newer Starfleet uniform design of the later movie, and James T. Kirk’s reluctance to accept a position at the admiralty rather than as a starship captain.
from the wikipedia page
This series has 6 books
1 - Wagon Train To The Stars
2 - Belle Terre
3 - Rough Trails
4 - The Flaming Arrow
5 - Thin Air
6 - Challenger