Skill Book
From Ymirheim Wiki
Skill books are in-game items.
To be able to learn new combat skills and level up already learned in-game characters' combat skills, players must use skill books. Skill books can be bought from Cattleya Organization NPCs, which are located across the in-game world. The books can be bought with the in-game Currency.
Each skill requires a particular skill book. Each skill level requires a corresponding skill book.
The skill books use a pop-up tip to describe some information about skills and skill books:
A player can buy any skill books, but can use only those that meet the class requirement. For instance, if the player has Knight as a current character class, the player can buy other classes' books, but can't use them.