Star Trek The Next Generation - The Q Continuum - Q-Strike
From the book jacket:

The mischievous creature who calls himself Q has subjected Captain Jean-Luc Picard and the crew of the Starship Enterprise to many of their strangest experiences. But little had been known of Q's curious existence or that of the advanced dimension from which he comes. But now Picard knows more than he ever dreamed about an ancient conflict whose consequences might spell the doom of the entire galaxy. 

The galactic barrier has fallen and Q's oldest enemy is free once more. Captain Picard and his crew find themselves in the middle of a cosmic war between vastly powerful entities. The future of the Federation may be at stake, but how can mere mortals turn the tide in such a superhuman battle? Picard has to find a way, or neither the Q Continuum nor the galaxy will survive.
Fiction, Jean Luc Picard (Fictitious character), Star Trek fiction, collectionID:TNG#, Fiction, media tie-in
Greg Cox
Greg Cox (born 1959) is an American writer of science fiction, including works that are media tie-ins. He lives in Oxford, Pennsylvania.

He has written numerous Star Trek novels, including The Eugenics Wars (Volume One and Two), The Q Continuum, Assignment: Eternity, and The Black Shore. His short fiction can be found in such anthologies as Star Trek: Tales of the Dominion War, Star Trek: The Amazing Stories and Star Trek: Enterprise logs. His first "Khan" novel, The Eugenics Wars: Volume One, was voted best sci-fi book of the year by the readers of Dreamwatch magazine. Cox can be found in a bonus feature on the "Director's Edition" DVD of Star Trek II: The Wrath of Khan.