Used for skill masteries
Skill Mastery are special abilities available only to those who have trained above 90 in a skill. Each skill has three abilities that vary in three intensity tiers. You need to obtain a Book of Masteries and the appropriate Mastery Primer for each skill.
Book of Masteries
Use a Contextual Menu on the Book of Masteries to open the menu to set your skill mastery.
In order to set a skill as your mastery your character must first read a primer for that skill. With the primer in your backpack, double-click to "read" and consume the book.
Primers have three intensity levels: Volume 1, Volume 2, and Volume 3, with three being the highest (best). The higher the level, the stronger the effect/ability your mastery will have.
Each mastery gives you three abilities. Usually one of which is passive and two that can be activated like any spell or Special Move. Similar skills usually share the same passive ability which is in effect as long as that mastery is selected. Activated abilities that are combat moves can not be used at the same time as a special move.