I read a lot of these back in the day, but found that the more it was opened up to more authors, the less worthwhile they became. By the time they started with the new jedi order books and other such expansions and spin offs from the prequels, i'd stopped reading entirely.
that said, the very first series lucas allowed other than novelizations of the movies* was by timothy zahn and i thought that triology was absolutely incredible. the first book was 'heir to the empire.' in all the books i read after that, while some were enjoyable, none ever came close to matching that level of quality to my mind. it's the original characters and set 5 years after return of the jedi. i especially liked the way luke was treated... he became far more interesting than the dour depressive of rotj. and grand admiral thrawn was one of the most compelling star wars villians not named darth vader.
i was also partial to michael stackpole's x-wing series, which were more action-oriented and had some great space battles scenes (which imho are very hard to do well). they didn't really involve the movie characters other than wedge though.
if you prefer the prequels and their characters and stories of the old republic and the jedi, i can't help you there. i never read any of those.
* there were a handful of books (a han solo trilogy, lando trilogy, and splinter of the mind's eye) released around the time of the original films that don't really factor into the explosion of the star wars mythos that zahn kicked off in the 90s. those books were more movie tie-in adventure stories. it was zahn's books that really kicked off a resurgence in star wars interest, being the only new star wars stories since return of the jedi... and they may have even been largely responsible for opening the door for the prequels.