Right now, the Age where I've made the least progress is Gahreesen. And since this is the 49th page, the next one will show up in the menu thingie on the left. The Gahreesen icon is pretty, so that seals that decision.
Before we get to that, however, the fifth slot to go into the leftmost part of the bookshelf is now full.
So I've now unlocked all the Ages we can expect on this quest, barring more side trips to dead ends. The DRC home base also didn't give me another book, so it probably is Balcony Age 2.
My focus, however, is on getting back into Yeesha's cave, and for that, we're off to the garrison. Charge!