This actually sounds like a Wizard reflavoring to me more than anything else. Instead of being able to script spells to their spell book they can perform a special ritual on a dead body to consume and trap some of it's spirit into a necronomican type book. That ritual has a cost in special herbs (the same as the cost of scribing a spell in a book), and takes as long as scribing the book. Also, it can be done with tokens from the defeated monster so that the wizard can do it during their free time.

If reflavoring makes it to easy to get normal wizard spells into the book, then you write a subclass. My own reading of Wizards goes like this.
2nd level - You get a combat type ability
2nd level - You can scribe from animal remains to add new spells to your spellbook.
6th level - Typically something not as directly related to combat
10th level - Helps in combat
14th level - Often a boost to a previous ability or other combat style feature.