The Sith Order began with a group of Force - sensitive beings who turned to the dark side of the Force. They were exiled from the Jedi Order or chose the path of the dark side on their own. The first Sith Lords developed their own teachings and ways of using the dark Force powers, which were often centered around domination and self - gain.
The novel of Revenge of the Sith is canon. It aligns with the established Star Wars canon framework and adds depth and detail to the events depicted in the movie of the same name.
Anakin Skywalker and Obi - Wan Kenobi are main characters. Anakin's fall to the dark side is a major part of the story, and Obi - Wan is his former mentor.
One of the best quotes is 'This is how liberty dies... with thunderous applause.' It shows the tragic irony as the Galactic Republic falls while the people, unknowingly, cheer for its downfall.