If you're looking for a decent sized series, I'd recommend the six book Errand series by Kevin Ryan. It's made up of two trilogies, Errand of Vengeance and Errand of Fury, which tell the story of the TOS era conflict between the federation and the klingons, split into basically four viewpoints - a starfleet command officer on earth, a security officer aboard the enterprise who is a klingon infiltrator, an officer aboard a klingon warship, and finally kirk.
One of the best flowing, and most satisfying trek stories I've read.
Book order if you're interested:
The Edge of the Sword
River of Blood
Seeds of Rage
Demands of Honor
Sacrifices of War