You can Enchant Books, and apply them onto armor that is available! (you no longer need to spend XP putting books onto armor/weapons) and these have a maximum set by the skill level.
22/44/74/112/158/212/274 Knowledge
+1/2/3/4/6/8/9 Max Combined Levels
Lapis Return
This Skill gives you the chance to refunded Lapis the more you spend!
18/45/83 Knowledge
+1/2/3 Xp Cost
+1/2/3 Lapis return
XP Return
This Skill allow you to be refunded XP per enchantment you make!