Core Skill - A skill that already has an initial maximum SP level of 10. No skillbooks are required to unlock these. You will be able to allocate points to these skills immediately after you advance to the fourth job.

Mastery Guide - A special, usable item that lets you increase the maximum SP level of a skill to 20 or 30, with a 70% and 50% success rate, respectively.
Non-core skill - A skillbook is necessary to unlock these skills.

Skillbook - An item which works like a scroll. This will allow you to allocate the first 10 points to a skill. These cannot be traded or put into storage.

Storybook - Special drop from non-boss monsters that allows you to begin the quest for a skillbook.

SKILLBOOKS cannot be traded/sold/bought.
skillbook = basic lvl 1-10 of the skill

MASTERYBOOKS/STORYBOOKS can be traded/sold/bought.
masterybooks = lvl 20, lvl30 skills books to upgrade with
storybooks = item used to unlock the basic lvl1-10 of a skill

Since only masterybooks/storybooks can be sold/traded, it means ANYONE can pick them up.