Barbara Hambly's
Ishmael is one of my favorites among the early Star Trek Pockets (#23). It's a bit obvious, and it makes the rather glaring error of calling Vulcan "Vulcanis", but it's delicately, beautifully written, a must for Spock fans. Good, old-fashioned time travel, better handled than such things often are (Terminator II, I'm looking at you); a strong understanding of the characters and relationships on board Enterprise; and some stellar OCs. It could be argued that it is fanfic, but if so, it's fanfic of the highest quality; and anyway, most of those early Trek novels are. The pre-Pocket ones, for instance, are an amusingly horrifying collection...
Part of the delight for me, in this one, is the inside jokes. Much like
Diane Duane, Ms. Hambly tips her hat to several other space-going stories--I can count at least three and there may be one or two I'm missing--but she does so in a way that doesn't interrupt the plot at all. Watch for them, and admire.