Why didn't he take the whole book?
It's not like he's stealing some specific pages without being noticed. He already went to the trouble of killing the librarian, but chose to leave the book only for someone like Strange to have a chance to beat him in future. Even if he couldn't foresee who would come after the event, there was absolutely no reason he didn't just grab the whole book. It's not like the book is too heavy to carry. And its hard cover can provide better protection to the bare pages. What was he thinking?