The Darth Bane saga

The Darth Bane saga by Drew Karpyshyn
A thousand years before the events of the Star Wars movies, the last Sith Empire was defeated...but one ambitious survivor saved the Sith Order by taking it underground, setting the stage for the rise of Emperor Palpatine and Darth Vader a millennium later. This is the story of that man, and one of the most raved-about Star Wars book series.

Buy from Amazon:
Book 1: Path of Destruction
Book 2: Rule of Two
Book 3: Dynasty of Evil

This is an Old Republic era series from the Legends timeline. It may be contradicted by newer books, movies, or TV shows (if so, just think of it as an alternate universe).