Expert skills are unlocked when an attribute (Strength, Intelligence, Piety, Vitality, Dexterity, Speed, or Senses) reaches 100.
Here is a link for some descriptions of the expert skills:
http://www.geocities.com/jandrall/Wi...8/Stats100.htm
Here is a link on what some of the expert skills give you, quantitatively that is:
http://www.geocities.com/shau185/attributes.html#expert
BTW, the descriptions of the skills are in the manual.