Previously, Schiller had transformed into the Black Sun, a mysterious being from the deep stars, listened to The Ancient One's reflections on that dream world, and written down all the worldbuilding related to the Great Old Ones in a book he gave to her.
The title of this book borrowed the name of the fictional tome Necronomicon, written by Abdul Alhazred, the mad poet of the Cthulhu Mythos system.
The Necronomicon did not exist in reality, but in the Cthulhu Mythos system, it was described as recording all knowledge in the world related to the Old Gods. Any investigator could find the forbidden knowledge they sought within it.
The Necronomicon written by Schiller was not quite so miraculous, but its contents did indeed include everything he knew about the Cthulhu Mythos system.
The Marvel Universe did have Cthulhu Mythos lore. Though it was not particularly orthodox and was closer to background flavor with the feel of supervillains, there was still a group of blind and idiotic mysterious beings waiting to return from the deep stars.
When Schiller gave this book to The Ancient One, he had merely hoped it would help the Sorcerer Supreme understand the power system inhabited by old enemies like Shuma-Gorath, so she would not be caught off guard.
The awkward part was that Eternity later rebooted the universe, and The Ancient One lost this portion of her memories. But because Eternity had not dared reboot anything related to Schiller, this book remained. The Ancient One was probably wondering where this book that had suddenly appeared in her possession had come from.
But Strange was even more bewildered than The Ancient One. His teacher had been studying a book that looked extremely dangerous behind his back, only to be caught red-handed by people from other universes present