Short answers, pulled from the story.
The first Star Wars spinoff material appeared in 1976 as a novelization titled From the Adventures of Luke Skywalker. George Lucas later commissioned Alan Dean Foster to write a sequel called Splinter of the Mind's Eye in 1978.
Timothy Zahn introduced this character in his 1991 novel Heir to the Empire which reached number one on The New York Times Best Seller list. The story follows Thrawn as he commands Imperial forces against the remnants of the Rebel Alliance using strategic brilliance rather than brute force alone.
On the 30th of October 2012 the Walt Disney Company acquired Lucasfilm for four billion dollars equivalent to about seven billion dollars today due to inflation. Subsequently Lucasfilm formed the Lucasfilm Story Group to keep track of and define canon across all media.
the 25th of April 2014 marked when Lucasfilm rebranded most Expanded Universe material as Star Wars Legends excluding The Clone Wars. This decision declared previous continuity non-canonical to shift focus toward new unified storytelling.
A New Dawn by John Jackson Miller became the first canonical novel published in September 2014 serving as a prologue to Rebels which aired a month later. The Force Awakens arrived in December 2015 sparking minor fan backlash against the restructured canon.