I think 'Reunion' is a great Star Trek TNG novel. It brings back some classic characters and storylines. It has a lot of the elements that made the TV show so popular, like interesting alien encounters, moral dilemmas for the crew to face, and a good amount of character development for the main cast of the Enterprise.
A really good Star Trek TNG novel is 'Dark Mirror'. This one takes the familiar characters and places them in a sort of alternate reality. It's fascinating to see how different they can be in this new setting. The plot is full of twists and turns, and it really keeps you on the edge of your seat as you read through it, exploring different aspects of the Star Trek TNG world.