Something apparently came up in the space between V'Ger and Khan that made it vital Kirk retake the Admiral's stripes and go do something else.
I like the idea proposed by some that Kirk had become Commandant of the Academy, which would be the easiest way of explaining his presence there and on a cadet cruise (the idea also proposes that by the time of TWOK, the Enterprise
was now a working cadet training vessel assigned to the Academy under Spock's command).
There are no novels between TMP and TWOK??
A small handful. Most of them simply have the Enterprise
conducting the same kind of missions she did during TOS.