The sixth Star Trek: The Next Generation book (although it's listed as number five because the first book, Encounter at Farpoint, is un-numbered). Written by Peter David and published by Pocket Books in 1989.
The plot involves a conflict between the Klingon and the Kreel, a primitive and violent species (think Vogons in THHGTTG). The Kreel have discovered a long-hidden cache of advanced weapons which threatens to disturb the balance of power in the sector. The Enterprise is sent to broker a peace deal between the two factions before a full-on war starts.
Typical Star Trek fare, really.
I think this is one of the funniest Star Trek books that I've read (although I can't say I've read all that many). In particular, I liked the mild innuendo between Worf and Gava (a Klingon woman) who is part of the Klingon delegation) which is a refreshing addition to the normally staid world of Star Trek.