Thrawn’s new trilogy concludes in this final new book from Timothy Zahn. The premise of this one is that Thrawn, while on an Imperial mission, runs into his own people, the Chiss. With loyalties divided between the Chiss military and the Empire, choices must be made. This book does have tie-ins with Star Wars: Rebels, even more so than the previous book. Thrawn: Treason takes place at the same time as season 4, episodes 9 & 10 of Rebels. If you have not watched the show in a while, this is the point where Thrawn is called away from Lothal for a few episodes.
Treason also features a proper introduction for the Chiss, properly reintroducing them into the Star Wars canon. This book shows audiences more about their technology, culture, and more. Namely, the fact that there are hostile races in the Unknown Regions being kept in check by the Chiss. It also features the return of Eli Vanto, Thrawn’s apprentice from the first book in the trilogy. Like many Star Wars materials taking place shortly before A New Hope (a la Rogue One), the events of Treason are closely linked to The Death Star.
The previous books already who Thrawn is and his integration into Imperial society. He is much more skilled at politics now than he was at the start. He is used to the less rigid structure of the Imperial military (compared to the Chiss) and the little games the leadership plays while trying to one-up each other. And still, years into his military career, Thrawn is repeatedly underestimated by his opponents. Both politically in the Imperial military and militarily by the alien threat being confronted in this story.
One interesting thing about this whole trilogy is the timing aspect of it. The first book came out before the 4th season of Rebels. Audiences had already been reintroduced to Thrawn and it filled out his backstory. But Alliances and Treason both came out after Rebels concluded. Anyone who has seen the show knows what happens to Thrawn later down the road. The events of Treason fill in a few holes from Rebels and leave a lot open to speculation about the future for Thrawn and related characters. On a final note, this book makes a lot more sense if you have watched Star Wars: Rebels to its conclusion. Doing so is highly recommended to fully appreciate Thrawn as a character.