Myst V is the latest game announced in the Myst series. At present, nothing is known about the game, including whether or not it is the same thing as "Something Else(tm)" (see below).
The latest project coming out of CyanWorlds is one of Cyan's best kept secrets. Much of what is being discussed on the Myst-related fora is rumor and speculation. However, according to sources at Cyan there are a few things that are known so far:
Something Else(tm) is D'ni related (source: RAWA)
Something Else(tm) has the Cyanists excited (source: RAWA, B. Slease, G. Dragon)
Something Else(tm) should come to light in a much shorter time than Cyan usually takes (source: B. Slease)
Novels
David Wingrove helped to write a series of novels focusing on the background story much more deeply than the games.
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The Myst franchise consists of a series of computer games and novels, centering around the storyline of Atrus and his family, who are descendants of the fallen D'ni civilization - a subterranean city whose people could link to other universes by writing a descriptive book about that world.
Sehv Tsahno - A Myst fangame dealing with one of Atrus' early Ages sabotaged by Sirrus and Achenar, and the unpredictable results of their writing manmade objects into that Age.
Search for the Springs of Kehlbet - A MYST fangame being developed by the Guild of Age Crafters.