Canon. Specifically C-Canon. And maybe T-Canon.
I've read the Darth Bane novels (Path of Destruction and Rule of Two) which heavily feature the philosophies of the Sith and their beliefs about power and controlling the Force, both during and after the New Sith Wars. Incidentally, those books are written by Drew Karpyshyn, the lead writer for the first KotOR video game, as well as Mass Effect. If you like KotOR, give those books a shot. They're quite accessible in that the time period they take place in is only covered in one other story, the Jedi vs. Sith comic miniseries (also worth reading). It's a very "fresh" era, and you don't really have to have followed any other Expanded Universe stuff to get into them.
Vaapad, Mace Windu's fighting style, is discussed at length in Shatterpoint, a Clone Wars novel that focuses on Mace (and is actually a Star Wars version of Joseph Conrad's Heart of Darkness), and a Star Wars one-shot comic that takes place during the Clone Wars about Mace Windu challenging Sora Bulq, one of his former masters and lightsaber instructor who'd fallen to the Dark Side.
T-Canon example of a non-Sith Dark Jedi: Asajj Ventress, the dual-lightsaber wielding apprentice of Count Dooku (Darth Tyranus). In the original Clone Wars animated series, Dooku specificially tells Asajj that she is not a Sith, despite being apprenticed to a Sith Lord, and he refuses to train her as a Sith.