Star Trek: New Frontier, book 01 : House of cards
Peter David1997

Sector 221-G: For the whole of Federation history, this large area of space has been controlled by the Thallonians, a cruel, militaristic race of which little is knownexcept that they rule the other races in their sector with vicious iron hand.
Now the Thallonian Empire has collapsed and the systems it once ruled are in chaos. Old hatreds are surfacing. Petty tyrants control deadly weapons. World after world is descending into disorder and self-destruction. The Federation must send a starship to help where it can and report what it finds.

That ship is the U.S.S. Excalibur, a newly refit Ambassador-class starship commanded by Captain Mackenzie Calhoun and manned by Starfleet's best and brightest, including some old friends from Star Trek: The Next Generation and some of the most dynamic new characters ever to crew a Federation starship.