Official information:
Official Thief Manual
Official Level/Title List
Official List of how to use skills
Other information:
Most of this information is for exclusive thiefs. If you are a member of more then one class, there will be different effects for some of these skills.
Information on using commands/skills for thiefs
Newbie class skills
Weapons and armour thiefs can and can't use
Items and their use (General for all players)
Maps of Malevolence (General for all players)
General information for thiefs