A Skill Book is the window that shows you your Skills, which you add your Skill Points to each time you level up. It starts with the beginner book, then adds the I, II, III, and IV books as you make the appropriate job advancements. The Skills for the fourth book, however, are only added after completing certain Skill Book Quests.
Beginner and Noblesse Skills
Beginner explorers and Noblesse start out with three basic skills: Three Snails, Recovery and Nimble Feet. They can only be leveled up to a minimal degree, for characters will gain new skills through job advancements. They also have Blessing of the Fairy; however, this skill cannot be improved through traditional means. Its level is instead based on the level of your highest level character on that server. Note that Legends, such as Aran and Evan, also start with these skills.
Citizens start with a different skill set. Also, unlike the other classes, they are able to raise all of their skills to their highest level.