Star Wars fans went through dark times between Return of the Jedi and the release of The Phantom Menace. For nearly 20 years, there really wasn’t much new content around.
Some relief came in the form of The Thrawn Trilogy, a series of books written by science fiction author Timothy Zahn. Set in 9 ABY, five years after the events of Return of the Jedi, the series chronicles the campaigns of Thrawn, the only non-human Grand Admiral and one of the best commanders in the fleet. He is the last of the Emperor’s warlords, and he takes the shattered remnants of the Imperial fleet and wields them into a force powerful enough to challenge the fragile New Republic. The series also features beloved original characters. Luke Skywalker is busy training new Jedi, and we even get to find out how Han and Leia’s marriage is getting on.
The first book in the trilogy, Heir to the Empire, was a runaway success, reaching #1 on the New York Times Best Sellers list on its release in 1991. Zahn was quickly hired to write two more novels — Dark Force Rising (1992) and The Last Command (1993).
But when Disney took over the franchise in 2014, it declared all non-film works non-canon. The popular Thrawn Trilogy was relegated to Star Wars Legends (the catch-all term Disney uses for non-canon material). Many fans were disappointed, so in 2017, Zahn was hired to write a new trilogy that fits perfectly within the Disney canon. These three books are Thrawn (2017), Thrawn: Alliances (2018), and Thrawn: Treason (2019).
So, the answer is that the original Thrawn Trilogy is non-canon, but the new Trilogy is written with Disney material in mind. Most fans agree that both series are good, but the original trilogy has the edge.