Crossroad, Dreadnought!, Black Fire, The Lost Years
and Forgotten History
are all excellent novels set between TOS and TMP. They're not all in continuity with each other, though.
The Wounded Sky
and My Enemy, My Ally
were originally set in a Star Trek: Phase II
(not the recent fan films, the original planned continuation of TOS) -esque second five-year mission *before* TMP, with a kind of halfway refitted Enterprise. ME, MA was later retconned into happening duting the original TOS five year mission and then re-retconned into having happened post-TMP by later books in the Rihannsu series.