A new race, called the "furies", make contact in the Alpha Quadrant with the Enterprise and a Klingon ship. Kirk must find if they are friends or will prove to be a threat.
First Strike was an excellent book that comes highly recommended. Every chapter in this book was meaningful. The action is constant -but not at the sacrifice of character development. The crew on the Enterprise were all well within character of what we have come to expect from them.
Like the TV shows, the "big 3" are in main focus here: Kirk, Spock and McCoy. However, there is another "big 3" that comes into play in a much bigger fashion. Captain Kirk, General Kellen of the Klingon empire, and Vergo Zennor, of the furies. The chemistry between the three "captains" - of the only three ships in this book - play off each other extremely well. While Kellon is looking for excuses to attack the Furies, Kirk sort of becomes the "referee", while he tries to access the possible threat himself...
The Furies are a very well developed race, and slightly morbid too. They are exactly what their name signifies.
First Strike starts a four-part "INVASION!" series that elapses from TOS, TNG, DS9 to Voyager. This novel is by far the best of the reading and one might want to consider stopping here. However, that may be difficult because the author sets you up for part two in a very good cliffhanger... but be warned if you decide to continue because the quality of writing drops... sharply.