Note: This review will contain spoilers for the original Thrawn trilogy.
Thrawn (the character) is one of the biggest fan favorites in the history of Star Wars. Author Timothy Zahn’s original Thrawn trilogy is considered by many fans to be the pinnacle (and start) of the old canon Star Wars Expanded Universe. Prior to that trilogy, all the “extra” Star Wars materials took place around the time of the original films. Zahn put out a series that was 6 years later and added a lot of new material to the franchise. The Rebel Alliance was the New Republic, Han and Leia were married and about to have a baby, the Empire was all but defeated, and more.
On top of all that he introduced Thrawn, one of the greatest Star Wars villains. Thrawn is loved because of his military genius, making him one of the few non-Force users to be a threat. If you are 3 steps ahead of everyone else, Thrawn is 5 steps ahead of you. That characterization largely remains the same in this new novel. Zahn seems to have done the best he can to keep Thrawn the same in the new canon. Thrawn now pops up before the original trilogy instead of after, but his role remains the same (crush the Rebels).
This book did come out after season 3 of Star Wars Rebels, where Thrawn serves as the new main villain. He was not in every episode there so we only received some sparse exposure to him. However, it was enough to show that Thrawn is still Thrawn and this book greatly exemplifies upon that.
Thrawn (the book) deals with how Thrawn (the man) joined the Empire and rose through the ranks. This is not a simple journey, as the Empire is largely made of anti-alien people. In addition to simply being an entirely new culture, the deck is stacked against Thrawn from the beginning. But like other villain novels, such as Tarkin, we see how the rise to power comes about. Whereas Tarkin was primarily ruthless, Thrawn is able to maneuver individuals like chess pieces. He does not start out this way; in the beginning of this novel, his skills for politics fail in comparison to his military genius.
Along with Thrawn, the book also develops some other interesting characters such as Governor Pryce (a secondary antagonist in Star Wars Rebels). While there did seem to be some restrictions on Zahn simply because of how the New Canon is set up, longtime Star Wars fans will be happy to see that Thrawn is truly back.