Truce at Bakura-It's OK. Basically some cool ideas but a ton of wasted potential. It takes place right after ROTJ as well.
Corellian trilogy-Sort of a 'meh' trilogy, but has some plot points used later on.
Shadows of the Empire-This is an interesting novel, it was basically a Star Wars film that was not a film-it had a soundtrack, figures, games etc. which all told a different POV of the story. Basically it takes place between ESB and ROTJ.
Darth Bane series-These novels are OK if a bit corny at times-they're basically a bit like the KOTOR games as they were written by the same guy....basically establishes how the Sith were wiped out and became "Always two there are".
Courscant Nights series-Haven't really read this, sort of got mixed reviews.
Republic Commando series-These novels are better than your average video game tie-in. The writer, Karen Traviss, however left when the novels conflicted with the Clone Wars TV show.
As for Union and the Mara Jade series, Union is very much an attempt at a more laid-back Star Wars tale, with the characters pretty much just having fun without any high stakes.By The Emperor's hand is a pretty dark tale written by Zahn and Stackpole.