I think you’ll find that a huge portion of the fan base credits the Thrawn series with extending the franchise beyond Return Of The Jedi. It had been long dormant until that novel.

For me, Zahn’s work is second only to the NPR version of Star Wars among my personal canon. After that, I’d definitely add the Daley trilogy of Han Solo novels. We have a thread dedicated to EU, with tons of reviews. I’ve put several myself, but I seem to remember they have mild spoilers. I’ll find some and check before I put in the link.