Thrawn is the only Star Wars character with two complete, separately published novel arcs written by the same author in two different continuities. That is a wonderful problem for a reader to have and a confusing one to shop for. Here is the order I would give a friend.
Start with the 2017 canon novel
Timothy Zahn's Thrawn was published by Del Rey on 11 April 2017. It is a clean origin story that assumes nothing, and it is the version of the character that connects to the Star Wars Rebels and Ahsoka television series. Follow it with Thrawn: Alliances and Thrawn: Treason to complete the trilogy.
Then decide whether you want the prequels
The Thrawn Ascendancy trilogy goes back to his life among the Chiss before the Empire. It is a very different sort of book — political, slower, more interested in Chiss family structures than in naval tactics. Read it if the first trilogy left you wanting more world, not more plot.
Then go back to 1991
Heir to the Empire (Bantam Spectra, May 1991), Dark Force Rising and The Last Command form the original Thrawn trilogy, now published under the Star Wars Legends banner. They are set after Return of the Jedi with a cast you already know, and they are the reason a Star Wars book shelf exists at all.
A note on order
Chronological order within the fiction and publication order do not agree here, and publication order wins. Zahn wrote the 1991 books for readers who had seen three films. He wrote the 2017 books for readers who had seen everything since. Reading them in the order they were written lets each one land the way it was built to.
The short version
- Thrawn (2017) — canon
- Thrawn: Alliances (2018) — canon
- Thrawn: Treason (2019) — canon
- Thrawn Ascendancy trilogy (2020–2021) — canon, optional at this point
- Heir to the Empire (1991) — Legends
- Dark Force Rising (1992) — Legends
- The Last Command (1993) — Legends
That is the spine. The Hand of Thrawn duology — Specter of the Past (1997) and Vision of the Future (1998) — and the Legends prequel Outbound Flight (2006) round out the shelf; all three are on our full Everything Thrawn track.