Just finished reading this new book: Thrawn, by Timothy Zahn (2017). I must admit that I was nervous about how the Thrawn mythology would be "re-invented" in a new, more Star Wars: Rebels way (read: changed for change's sake; silly disregard for previous tales).
To that end, I was quite pleased. For a EU veteran like me, this book fits seamlessly into the rest of the Thrawn information. It does not, as near as I can tell, change anything so much that the Thrawn Trilogy would be rendered incorrect.
One minor gripe with this book might be just that: it feels a bit...minor. It has some decent action, near the end, but no good guys to follow and for whom to root. Remember, Thrawn is a bad guy. An interesting and enlightened bad guy, but still a bad guy.
We learn Thrawn's origin story, following a short re-telling of the events of Mist Encounter, the Zahn short story (from Star Wars Adventure Journal Vol. 1, No. 7, WEG 1995). We then see Thrawn attend the academy, Thrawn hit a low point in terms of dignity (he disguises himself with dark glasses and a robe to meet some low level flunky in a diner. Oh, the humanity!).
Overall, I am pleased. Not overwhelmed, but pleased with this book. I am very glad that Disney didn't hire a Star Wars great like Zahn to simply ruin his own greatest creation with a needless and oddball revision.