Spellbook

You have figured out how to copy spells into a spellbook.  You aren't very  good at it as you were not trained as a wizard, but you at least have figured it out.

You can copy spells to the spellbook as per the rules for a Wizard and are able to change your prepared spells from them.   Becasue you were not properly trained it takes you longer and costs you more money.   Double the time and double the gold cost per spell you copy into your book.   You must copy a spell from a scroll or another spell book, it can't be a spell that you just know.