Riven, Stranger

Atrus and Catherine had previously trapped Gehn on Riven by removing all linking books from that Age; the very last book to be removed, linking to the island of Myst, was the one they held to escape Riven. Atrus hoped it would be destroyed; however, it fell into the Star Fissure, a void leading out of the reality of the damaged Age of Riven and into unknown space. Catherine was later tricked into returning to Riven by her sons, and was captured by Gehn. Subsequently, the Stranger discovered the Myst book, sparking the events of Myst and, some time later, Riven.

Also, he does not want to risk sending a real linking book to Riven until Gehn is safely imprisoned, in case his father should capture the book and use it to escape his confinement: the Stranger must enter the Age with no way of leaving.

When it opens, Atrus immediately links to Riven to investigate, and meets the Stranger at the brink of the Fissure. Depending on the player's actions, the ending to Riven varies. In the canon ending, the Stranger tricks Gehn into the prison book and releases Catherine. Atrus and Catherine thank the Stranger, before linking back to the Age of Myst. The Stranger then falls into the Star Fissure, to be taken on the path back to his world. Different player actions can result in the Stranger's entrapment in the prison book, or even his death.

Source: Wikipedia > Riven