Board Thread:Consensus Track/@comment-187.23.113.8-20140202201034/@comment-24511216-20140206052050

Using the Black Book resets your perks in a skill tree of your choice at the cost of one dragon soul. Making a skill legendary resets the skill tree back to 15 and refunds all the skill points you put in the perks.

Resetting skill trees allows you to continue training so you can increase your character level past the normal cap of 81. Any maxed out skill tree can be made legendary even if it has been done before.

I got a character to level 147 before I messed up on a quest and was forced to delete him.