The Blue Book (along with it's twin the Red Book) is a Prison book written by Atrus with the intention of trapping greedy explorers within them. Atrus altered the book to show an age full of riches, however, when someone links to the age, they become trapped within the book until someone else switches places with them. In Myst, Achenar is trapped within the book and requests you find the blue pages to repair it and release him. However, if The Stranger does so, they become trapped within the book and Achenar gloats as he tears the blue pages out, leaving The Stranger trapped within.