It would make an awesome reward for getting through an Age - being able to control random tricks like that (hidden speakers, imager illusions, fog machines...). It would work best in some kind of illusion based Age, I think, and would make sense in a training area.
The solution might be to work through the illusions and reset them from a control room so they fit their location. Meaning that a water sound should be keyed into a speaker near water, etc. Sort of like a reverse Selenitic Age. Or maybe by changing the sounds in each area, the player could change what actually appears in that area. Switching a water sound to a wind sound might shut off a stream and open a cliff so that the wind could sweep through an area, making the sound you input equal to what you'll find when you go to that area with the associated speaker.
But I don't think the D'ni needed fog machines, unless they used those and fire-marbles and held rave parties.
what would be on the pages?
Unreadably small text like in other Myst games. We don't know enough about the Art to know what would belong on the pages. But to differentiate the pages, we'd have to make the script appear different on each. Maybe you could just take "lorem ipsum," put it in D'ni script, make the font size really small and break up the paragraphs and indentation to make different page graphics. At the bottom of each page you could have a D'ni number so that you could differentiate between the pages that you've found.
A book, a couple logical steps, maybe some artwork based clues - that would be better, I think, so it's not always just "go back to the book again for the next step".
Yes, but the reader should be able to get the gist of what's going on from just one reading of the book, since we'd have to make the theme pretty evident. And none of the puzzles would have to rely on cryptic passages that nobody would get, but I think that reading a book should be mandatory to solve one of these ages.
with various D'ni or explorer artworks and poetry scattered around
That would be a great idea, since if we're going to be able to add fan-made ages, then surely Cyan wouldn't have a problem with including other fan-made material.
--Equinox