The Expanded Universe was always enjoyable to read, maybe not the greatest works of literature ever, but you got to see how life was progressing post-Return of the Jedi, you got to see how Luke, Leia, and Han's family and friends lived and died.

There were some pretty good authors involved over the years too, like R. A. Salvatore, Timothy Zahn, Kevin J. Anderson, Michael Stackpole, Karen Travis, William C. Dietz, Kathy Tyers, among many others. If you read any other seriailized series like the D&D novels, Battletech/Mechwarrior, Star Trek, Warhammer, and all of the many others that exist, you will probably recognize some of the authors as writers in those series as well.

As for the Prequel books and other novels post cancelation of the Star Wars Expanded Universe there was some quality improvement per novel as the universe became more definitive and more in line with the cinematic universe.

As a lifelong Star Wars fan who read the EU books for around twenty years, I'll say I miss the version of Star Wars they were building, it wasn't always the heroes winning the day, there was plenty of heartbreak and betrayals to go around, but it was familiar and enjoyable.

Maybe? There are a lot of them and the quality varies depending on the book. You can't go wrong with the original Thrawn Trilogy by Timothy Zahn. Heir to the Empire, Dark Force Rising, and Last Command were considered the unofficial sequel trilogy by some fans and they started off the entire Star Wars Expanded Universe, which got rebranded to the Legends continuity when Disney took over. Besides the Thrawn Trilogy, I'd also recommend the Revenge of the Sith novelization. I went in not expecting much, but it really expands on a lot of things the movie brushes over, and witnessing Anakin's story from his viewpoint was a lot more compelling than what we got in the movie. From there, it really depends on what you want to read about.
The Star Wars book finder is a neat tool that lets you search by author, topic, or genre if you're looking for something specific.

I've enjoyed dozens of them over the course of my twenty something years being into SW. Currently I'd recommend the Plagueis, Tarkin, the Thrawn books, and the Bane trilogy. Apparently I like villain books. I've also enjoyed the Karen Traviss Clone Trooper books but they got cancelled for canon reasons due to the TCW series.

Authors have plenty of freedom unless you're talking about novelizations of the films.