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Once you have collected up several necromancy scrolls, you need a Necromancy spellbook, yes, only one book is sufficient. It is called the Codex Damnorum and can be found on Mages, Adept Mages and Necromancers. You use the inscription skill on the necro scroll and then target the Codex. There is a magery check as well as an inscription check. You will either succeed or fail. If you succeed, the spell is put into the book. If you fail, you will lose the scroll. I recommend GM scribe and GM magery to do this as finding these scrolls are difficult. You cannot "copy" these spells from a book into a new book, you can only find the scrolls and inscribe them into the book.