This achievement is unlocked by returning all five blue pages to Achenar. Four are found in the ages (one each) and after bringing all of them back to Achenar, he'll give you instructions on how to find the fifth one.
*** Spoiler - click to reveal ***The fifth one is located behind the fireplace in the library, along with the final red page and a mysterious green book. Achenar will give you instructions on how to find the correct pattern to unlock the secret compartment after bringing him four pages. Make sure you save before returning the fifth page to Achenar (trust me) and after doing so, you'll unlock this achievement.