Under the "Player>Books>Skills" menu players can view their current collection of stolen skills. This is a public list, so even your friends can view yours if you link them to it.
Your skill book, like other forms of storage here, has a limited number of slots. You can expand these slots with the Credit Shop item Skill Book Storage, which you can find right here.
You can expand the number of skills in your Skill Book by buying them from Skill Shops and stealing them from enemies.
Please note that adding Extra Skills to your Skill Book from within the Skill Shop will be unavailable to you until you have a creature that is level 50 or higher.