There were hundreds of books in the library that all told Leo what the author thought about magic. In the end, that was what books were.
They were just what the author felt was right. The only thing that made it believable was the credibility of the author. In the case of books about magic, this credibility came from magic power.
All the books that Leo was reading were copies of journals of 7th Circle and 8th Circle Mages. These were only reproductions, but the knowledge was priceless to Leo. As he read book after book, he started to see some similarities between what each book told.
As he started making concrete connections between the books he was reading, he started reading with a stronger fervor. He had long gone into a trance as he devoured the words coming out of the books. As the time passed, his reading speed increased instead of decreasing.