the old extended universe (the Lucas era) was very diverse and quite adventurous.

There is Zahn's Heir of the Empire series about Admiral Thrawn coming back from his Star Trek mission to outer worlds and kicking New Republic's ass so hard it wasn't even funny - gold standard basically - should've been sequel trilogy if the world was fair.

Then there is Shadows of the Empire - the midquel set between Episodes 5 and 6 - one of the early glimpses into Star Wars underworld and Palpatine's involvement with space mafia.

Kevin J Anderson's Jedi Academy is basically sequel trilogy as god intended - it's about Luke reestablishing the Jedi Order and the many challenges he faces. There's also my personal fave I, Jedi by Michael Stackpole who did the X-Wing series which is also god tier - it is about undergoing jedi training told in first person.

Alan Dean Foster's Splinter of the Mind's Eye is alternate Episode 5 (Dark Horse redid that story retconing it back into official continuity as 4-5 midquel) and since its a Foster book it is smooth as morning mist.

The New Jedi Order series is basically the sequel sequel trilogy with new threat invading the galaxy, it's hit or miss most of the time, but they take some giant swings in this one so gotta respect that because the best Disney Star Wars novels are Zahn's rewrites of Thrawn novels.

There's also the whole Dark Horse era of comics which is all over the place including Yoda young romance limited series and a crossover with Aliens (featuring Boba Fett doing god's work among other things) Marvel Star Wars been trash and remain trash, then now and forever - started strong, got caught up in retconing the shit out of itself when 5 and 6 came out and then completely lost its plot and died a death of irrelevance - Dark Horse kept it self-contained limited runs that moved fast and hit hard.