Sorcery Book

"And this was found in Gaius's chambers?"

- Arthur to Agravaine

The Sorcery book is the book that Agravaine planted in Gaius' chamber so that he would get condemned for practicing magic. Later he showed the book to Arthur who was clearly upset of the thought of Gaius betraying him. He later told Merlin while he was showing him the book that Gaius had condemned himself. Later when Merlin found other books like "Studies in Sorcery" hidden in Agravaine's chambers.