Choices of One

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.

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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).