^There are a lot of older novels that are nominally set in the interval between TOS & TMP -- including just about any TOS novel by Howard Weinstein or L.A. Graf -- but few of them really do anything with the period that couldn't have been done in a 5-year-mission novel. The Graf books do focus heavily on Chekov as security chief, so there's that. Weinstein's Deep Domain
offers a version of Kirk's decision to give up the ship again and go back to the admiralty. The two early Pocket novels by Sondra Marshak & Myrna Culbreath, The Prometheus Design
, are more heavily rooted in exploring TMP concepts than most Pocket novels, but the way they do so is, well, not that well-regarded.