Books of Sorcery

"And this was found in Gaius' chambers?"

- Arthur to Agravaine

The Books of Sorcery were books of magical knowledge that Agravaine used to frame Gaius.

History
After Morgana hired Alator to kidnap Gaius, Agravaine planted a book of sorcery titled Witchcraft, Sorcery & Magic in Gaius' chambers to make it look as though he'd turned traitor and run away.

When Agravaine presented the book to Arthur, he was deeply upset and refused to listen to Merlin's assertions that Gaius would never do what he had been accused of. He slapped the book down on the table as he left the room.

Merlin later searched Agravaine's chambers for proof that he was involved in Gaius's disappearance and found several other volumes - including one titled Studies in Sorcery - hidden in a small chest beneath his bed (The Secret Sharer).

Trivia

 * The Art Of Necromancy bears a strong resemblance to these volumes, which may imply that they're part of the same set (The Secret Sharer, Lancelot du Lac).