I am a huge Star Wars nerd especially some of the older novels that are now considered not part of canon.
One of my favourites was Darth Bane and the three set series that helps to make the Sith and the Rule of Two seem sensible.
The first book Path of Destruction you meet Des or well as he later is called Bane. A willful youth who made mistakes and had to flee his home on Apatros where he was basically a slave to a mining company.
When he runs he joins the Sith Lord army of darkness. He outshines everyone that he is with showing how strong and powerful he really is. Then he shows that he had the innate ability that allows him to access the force.
The story shows a lot about evil and how and what it is. It also shows that not all evil is mindless hate but can actually be of some use. Bane is not evil as some but he really does use a powerful way in which to further his own goals.
I really fell in love with this book because I always like to see the buildup of what caused the powerful and secretive nature of the Sith.