The Final Reflection by John M. Ford
The first of two must-read Star Trek novels by Ford, The Final Reflection is the first novel to delve into Klingon culture and show the Klingons as having honor and valid ideas of their own. Ford invents a Klingon version of chess, called klin zha, and uses it to explore the Klingon view of the universe and their place in it. Pretty much all of the later development of Klingons in Star Trek builds on Ford's work here, and it's also a great piece of world-building and a tutorial in how to make an alien race feel alive and real. For the rest of his life, Ford was feted by Klingons at conventions as their great hero.
How Much for Just the Planet? By John M. Ford
Having given the Klingons a new depth and integrity, Ford chose to follow that up with... a musical comedy book. The Enterprise travels to a Dilithium-rich planet, where the Federation is competing with the Klingons for the mining rights. And weirdly enough, everybody on the planet communicates by bursting into song. And comedy is king. This book features cameo appearances by Pamela Dean, Neil Gaiman, Diane Duane and other authors.