The Book of Oa is a massive tome kept on the planet Oa.
The book contains billions of years of knowledge, including the history of the Guardians of the Universe and Green Lantern Corps. As the Green Lantern Corps were dispatched to Maltus, Hal Jordan consulted the Book of Oa and looked at the star systems absorbed by Aya. Tomar-Re noticed 2813, the space sector he was assigned to, was part of the map and commented it was devoid of life before Aya. On a whim, Jordan compared the map with sectors that are uninhabited and discovered Aya absorbed only sectors with no lifeforms in them.
- 126. "Dark Matter"