Wasn't Gehn some kind of nobility/royalty?
It seems that they had a normal economy, even with books, in which artisans (crafters, cooks, architects to build in D'ni, etc) and farmers were still needed, just not so much raw resources (I don't think). Books were also sold, like real estate, and most people didn't own books- they still had to buy materials they needed.
Anyway, I'm pretty sure there was a 'D'ni money' and that it was mostly used to buy things that were built- money is just a medium of rating how one thing should be exchanged for another thing, and they still exchanged things in D'ni. At the very least, written books take time to craft (a novel you buy is not priced for the paper and ink it is written with, is it?) and they used magic ink and paper or something, which I'm sure was limited in some way.
Atrus (and probably other age-writers, who were a small upper class, and very distinct from commoners) seems to have "homesteaded" by writing and maintaining Myst, Tomahna, and the Cleft entirely by his own toil, just as a frontier family of yore who made almost everything they needed themselves.
That said, when he did need money (IE, food before he wrote his home) I think Gehn's deep pockets provided it. It doesn't seem that Atrus ever had a job, though it's not impossible- hell, maybe he used to work part-time making hamburgers for the guild of caterers.
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