Reddit reviews Allegiance (Star Wars - Legends)

I've listed it before, but Allegiance by Timothy Zahn. It has the principle cast of A New Hope and also revolves around a band of stormtroopers that are disillusioned by Alderaan getting destroyed.

If you've wondered what a kickass light-side Imperial Agent looks like, or a Trooper who's not willing to toe the party line and become a white-armored thug like all the others, Zahn's Allegiance is one of the best books I've ever read. The sequel Choices of One is also excellent.