So I've been thinking about this for a while and I was thinking of what would be some of the resources left behind for the GoW other than the Rehevekor.
First there arethe resources we know we have, the linking Books in our Relto's and in the City, They may not have a layout of entire Ages but they may have pieces that would be useful to us... like training wheels.
Secondly while I know that the Myst books are not entirely canon in the very first book it mentions that there are copies of the Descriptive Book of Earth. I would imagine that if we found one of these it would be immensly helpful to us because unlike any other Age we Humans have been studying our world for thousands of years, we should have a better grasp on it than any other Age and so should be able to understand the structure of that Descriptive Book faster than others.
Thirdly discovered Descriptive Books, this one in my opinion may be difficult, it seems to me that for the most part D'ni was largely cleaned out of Ages, after the D'ni people left for Reeleeshan. Leaving (and this is my guess) only three groups of Books.
1. Some left behind. While the D'ni were thorough I see no reason why .01 percent of the 10,000 Books they once had could not have been left behind or overlooked.
2. Illicit Books. Unstable, flawed, illegal Books hidden throughout the City. The D'ni wouldn't have been looking for them as they left meaning there could be bunches hidden all around the city, possibly more than were left behind. Some could even be thousands of years of and forgotten but the Earthquakes opened their 'tombs'
3. The Tomb of the Great King,I suspect the D'ni left these behind, I also suspect the DRC has claims to all of these Books (If Pento is any indication)
So these are my thoughs make of them as you will I hope they helped.