Board Thread:Consensus Track/@comment-187.23.113.8-20140202201034/@comment-25075055-20140202202238

With the black book, you can reset it whenever you want and it doesn't reset your skill all the way back to 15: it basically means you can instantly refund your perks with no change in skill level and use them elsewhere. For example, say you put perks into the Destruction tree, but you don't use destruction spells anymore: the black book will refund all perks you've put in that tree, allowing you to use them elsewhere on a skill tree you actually use.

Overall, refunding perks from the black book is a lot better than making something legendary.