That wasn't my understanding when I reread the appropriate passages in Wagon Train to the Stars this morning - my understanding was that Kirk had already been promoted back to the admiralty (and Spock taking the center seat); Kirk stepped back aboard the Enterprise as captain only for the duration of the Belle Terre mission.
The thing is, I don't know that it was ever an issue of his being promoted again. He was only demoted following the events of The Search for Spock
. At the outset of TMP, he accepted a grade reduction to captain so that he would be allowed to command the Enterprise
during the V'Ger mission (hence the change in his uniform and rank braids) and (allegedly) for a time thereafter, including the Belle Terre mission.
Other novels have him commanding Enterprise
on a mission to something called the Aquarius Formation. Ship of the Line
takes place in 2278 and states clearly that he's returned to the admiralty by that point, as Spock has replaced him as Enterprise
captain following the ship's assignment to Starfleet Academy.
Kirk is in command of the vessel (and Spock is first officer) for a brief cameo because the Enterprise
has been temporarily returned to active duty due to the emergence of a Klingon threat near the Typhon Expanse, but the novel's text makes it plain that Kirk has joined the faculty at the Academy and is no longer commanding the vessel on a regular basis, similar to the arrangement he has at the start of TWOK.