Choices of One by Timothy Zahn
In this sequel to Timothy Zahn's novel Allegiance, Mara Jade and the Hand of Judgment are sent on a mission to prevent a traitorous Imperial governor from allying with the rebels - even as Luke, Leia, and Han are on their way to meet with that very governor. Looming over the whole thing is a threat from a mysterious warlord from the Unknown Regions, whose power might take both Rebel and Imperial courage and brains to overcome.
A younger version of fan-favorite character Thrawn - not yet a Grand Admiral - plays a role in the story, and this book is also a must-read for any fans of his future right-hand man, Gilad Pellaeon.
Buy from Amazon!
This is a classic era book from the Legends timeline. It will likely be contradicted by newer books (if so, just think of it as an alternate universe).