This achievement will unlock upon returning all five red pages to Sirrus. There are five total: four in the ages with the fifth
*** Spoiler - click to reveal ***hidden behind the fireplace in the library. Sirrus will tell you how to find the combination after returning the four pages to him which after using will open the fireplace and give you access to the final red and blue pages (as well as a green book but ignore that for now). Save your game (trust me) and give Sirrus the final page to unlock this achievement.