If someone wants more info on Dooku and doesn`t want to pick the book, here is a scene that was cut from the script:
"The bronze bust of an elderly bearded man stands among a line of other busts of Jedi in the Archive Room. Obi-Wan stops in front of it, studying the striking features of the chiseled face. On the walls, lighted computer panels seemed to stretch into infinity. Farther along the room in the background, five Jedi are seated at tables, studying archival material.
Obi-Wan studies the bust for a few moments before Madame JOCASTA NU, the Jedi Archivist, comes up and stands next to him. She is an elderly, frail-looking human Jedi dressed in a yellow and brown robe. She’s tough as old boots and smart as a whip.
JOCASTA NU: Did you call for assistance?
OBI-WAN: (slightly distracted) Yes…yes, I did.
Jocasta Nu notices the bust Obi-Wan’s studying.
JOCASTA NU: He has a powerful face, doesn’t he? He was one of the most brilliant Jedi I have had the privilege of knowing.
OBI-WAN: I never understood why he quit. Only twenty Jedi have ever left the Order.
JOCASTA NU: (sigh) “The Lost Twenty”… Count Dooku was the most recent and the most painful. No one likes to talk about it. His leaving was a great loss to the order.
OBI-WAN: What happened?
JOCASTA NU: Well…Count Dooku was always a bit out of step with the decisions of the Council. Much like your old Master, Qui-Gon Jinn.
OBI-WAN: (surprised) Really?
JOCASTA NU: Oh, yes. They were alike in many ways. Very individual thinkers, idealists…
Jocasta Nu gazes at the bust as she speaks.
JOCASTA NU: He was always striving to become a more powerful Jedi. He wanted to be the best. With a lightsaber, in the old style of fencing, he had no match. His knowledge of the Force was…unique. (beat) In the end, I think he left because he lost faith in the Republic. He believed that politics were corrupt, and he felt the Jedi betrayed themselves by serving the politicians. He always had very high expectations of government. He disappeared for nine or ten years, then just showed up recently as the head of the separatist movement.
OBI-WAN: It’s very interesting. I’m not sure I completely understand.
JOCASTA NU: Well, I’m sure you didn’t call me over here for a history lesson. Are you having a problem, Master Kenobi?"