The Red Book (along with its twin the Blue 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, Sirrus is trapped within the book and requests you find the red pages to repair it and release him. However, if The Stranger does so, they become trapped within the book and Sirrus gloats as he tears the red pages out, leaving The Stranger trapped within.

Red Book & Spire[]

It is often disputed whether or not the Red Book is actually Spire or merely a prison age that held Sirrus until he was transferred to Spire. In the events of Myst (Game), the Red Book is shown as a dark void with no scenery. Atrus disguised the linking panel to show an age full of riches to lure greedy explorers who happen upon his linking books (though they ensnared his sons and they became trapped). However, the trapped individual can see anyone who opens the book and interact with them, so long as there are no missing pages (which causes static that interrupts the linking panel, blocking both parties ability to see and hear the other). Should someone attempt to link to the age while someone is in the book, the two swap places and the linking person gets trapped within the book while the trapped person is freed (as seen in Myst's bad endings). In Myst's ending, Atrus (supposedly) burns both the Red Book and Blue book, which leads players to believe they died.

However, the events of Myst IV: Revelation seem to contradict this view. Though it's not explained within the game whether or not he actually burned the books back at Myst, he admits that he left Sirrus and Achenar imprisoned to see if they'd change their ways. He traps Sirrus within Spire and Achenar within Haven. However, various clues within the game lead the player to believe that these two ages were in fact the earlier Red Book and Blue Books from Myst, contradicting Myst's description of these books. Further contradictions are the ability for multiple people to link within the ages and the ages not being large voids.

Fans have speculated whether Atrus rewrote the two ages after the events of Myst to accomodate his sons, whether Atrus transfered them to the two ages or to simply ignore Myst's description of the books entirely.  However, there is sufficent evidence in both Myst and Myst IV that suggest that either scenario could be true: it could be the same age or two seperate ages.

Richard A. Watson, however, has confirmed that the Red Book is indeed Spire.  He states that the Myst team did not want to reward players with a new age by getting a bad ending and as such left it out [1].  He states that much of the Myst and Riven's story is altered for the sake of playability and that many points contradict the game (such as their imagined Myst Island being much larger and Atrus having many more hiding places for ages).