HiroshiMishima: Yes, I much enjoyed how Riven continued where Myst left off. But it's also understandable why Myst never mentioned Riven originally, seeing as how they hadn't planned to make a second game and so never said in-game what Atrus was doing or how his sons tricked him.
*SPOILERS*
Riven's name is not mentioned but the Age itself is mentioned in Myst.
From Atrus' final speech in K'veer:
"Hmm, it is done. Oh, I have many questions for you, my friend, but my writing can not wait. I fear that my long delay may have already had a catastrophic impact on the world in which my wife, Catherine, is now being held hostage."
Add that to the bit POLE7645 quoted, also from Atrus' final speech in K'veer, and It looks like they already had at least a general idea on where they wanted to go with the plot on a second game.
You're also told how Sirrus and Achenar tricked Atrus.
From Atrus' speech in K'veer, green book :
"They devised a plan - an evil plan. I had no idea to what extent their greed had progressed. Their own mother... they used their own mother (oh, my dear Catherine) to lure me here to D'ni. Of course I could return to Myst, except that they removed a single page from my Myst linking book. I cannot return without that page."