Treatises are books that allows the players to unlock new Skills when read. Through Treatises you can unlock all possible skills on any single character. Some characters start with several skills already unlocked but others still locked. These characters will also have to unlock the remaining skills by reading the appropriate Treatise (1).
Sorcery skill trees are divided into several Treatises that unlock different parts of the skill tree. Even if you find a Treatise of a higher rank first (like the Geomantic Treatise III), you can't put skill points into those new skills until you also learn the skills from a lower rank Treatise (like Geomantic Treatise II) and put skills into them first (2).
Treatises can be found randomly in the dungeons or sold by the merchants Gerlot in Mannshire and another merchant in Brynn. Some Treatises can drop only from specific enemies, for example the Pyromantic Treatise IV drops only from Bandit Pyromancers.
List of Treatises
Page last edited during patch: 0.6.1.10