The Book of D'ni is the third and currently last (two more books are planned to be released in the next few years, one of which will be known as The Book of Marrim) part of the Myst series of novels, set in the same universe as the Myst computer games by Cyan Worlds and written by the Miller brothers, creators of the Myst game.

In the Book of D'ni, Atrus, the last D'ni known to be alive, together with his wife Catherine and young helpers from an Age he wrote (Averone), finally manages to open the crumbled exit of the room in K'veer and gain access to all D'ni.

They then set out to find old linking books and find D'ni survivors who eventually escaped to, during the Fall. After they indeed find survivors, like Tergahn and Tamon, they start planning to rebuild D'ni together, but this is put to a halt as a link to a great lost Age is found: Terahnee. Terahnee turns out to be a sister-world to D'ni, populated by a people who, like the D'ni, derive from the Ronay.

Atrus plans to move there with all the D'ni survivors, but as he makes a horrible discovery about the Terahnee culture he instead wishes to leave. These plans are cut short as a plague strikes Terahnee, destroying Terahnee as D'ni was destroyed before.

In the D'ni timeline, the Book of D'ni is set after the events of Myst and Riven, but before Myst III: Exile. The book discusses the early history of the D'ni, something which is further expanded on in the game Uru: Ages Beyond Myst and its expansion packs.