First off, steer clear of Ed Greenwood. Literally one of the worst authors alive. He's good at doing D&D game stuff (he's the creator of the Forgotten Realms) but he cannot tell a story to tell his life.
As for the Star Wars books, there's a lot more than 21 of them. I think at last count there were about 100. Some of them are very good, some of them are awful and most are okay-but-a-bit-meh. The earlier books published by Bantam got a bit old very quickly, as they kept dredging up repetitive plots (usually revolving around a random superweapon of the week being deployed by a renegade Imperial admiral to destroy the New Republic). Of the Bantam books the
Thrawn Trilogy is highly recommended as it is written by a proper SF author (Timothy Zahn) and captures the spirit of the movies well. The
Corellian Trilogy is also okay, with some nice ideas in it. The
X-Wing Series is worth a look as it relegates our main heroes to the role of supporting guest cast and introduces a new regular cast led by Wedge Antilles. Some of these books are pretty good and the new characters are worthwhile.
The New Jedi Order, published by Lucas Books, is a much more original and interesting project. This series consists of 16 books divided into several lesser trilogies and duologies and single books. The theme of this series is that 20 years after
Return of the Jedi the Star Wars galaxy is invaded by an incredibly aggressive and powerful species known as the Yuuzhan Vong (no laughing at the back). Wielding organic technology, the Vong are strangely immune to the Force. Aware that they do not have enough troops to conquer the entire galaxy, the Vong only secure important worlds, obliterating others with their powerful gravity-based weapons, and direct their forces to destroying the New Republic and the Imperial Remnant to become the dominant force in the galaxy. Several characters from the films (including one of the 'big ones') die during the course of the books, and several major characters established in the Bantam novels also buy the farm. The NJO is much darker and much grittier than anything seen before in the Star Wars universe.
The books are reasonably entertaining and more consistent than the Bantam novels. Only one is really bad - Elaine Cunningham's
Dark Journey - and most are above average. The series is worth reading for two books:
Star by Star by Troy Denning (since RA Salvatore lost the plot, the Forgotten Realms' best author and snapped up to write
Star Wars books and even become one of the directors of the line), which will have your jaw buried somewhere in the ground.
The other reason for reading the NJO is
Traitor by 'proper' SF and fantasy author Matt Stover.
Traitor is the best piece of
Star Wars in existence aside from
Empire Strikes Back and
A New Hope, bar none. It's very different (a psychological thriller of sorts), addresses the question of the Force in a way that's genuinely intriguing and not boring with cod-philosophical ramblings, and has the best lightsabre scene in any of the books. Simply excellent.
There have been two series set after the NJO, the
Dark Nest Trilogy and now the
Legacy of the Force series, neither of which I've read, but both have gotten good reviews.