how about a feature to trade skill books with NPC for the colour you needed. eg. 5 of lvl 1 red for 1 lvl 1 blue? do correct me if i''m wrong, all mage need blue, warrior need red, rogue need yellow. so if we do not need the other books, isn't it better to trade for the colour you require. and selling to shop will be such a waste as skill books (of the correct colour) are so hard to get
 "Sword & Magic"){kind=icon}
 "Ninja Warz"){kind=icon}