Thrawn Ascendancy: Lesser Evil by Timothy Zahn
The adventures of young Thrawn conclude in this third book of the Ascendancy Trilogy, the prequel to the canon Thrawn novel. The trilogy tells the story of Thrawn's history in his home region, the Chiss Ascendancy of the Unknown Regions, before joining the Empire.
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This is a book from the New Canon timeline! It's guaranteed to be consistent with future movies, TV shows, books, and comics (and almost all new stories since mid-2014).