Signs are certainly a very frequent recurring motif author Jeanne DuPrau uses in The City of Ember. One of the most important signs is the "E drawn with curving lines" on the surface of a stone in a cluster of stones along the river in the Pipeworks, right where the "river disappeared into a great hole in the Pipeworks wall" (Ch. 14). Lina and Doon have figured out that this E stands for egress and leads to the way out of Ember.
In chapter 15, after Lina and Doon find the door leading into the boat room, the candles, and matches, they discover other important signs. The one boat in the first room and the metal box is labeled with signs, and Lina assesses that the signs are teaching tools to help the citizens understand the directions on the piece of paper Lina found:
We learn what's what on the one that has signs. "Boat." "Paddles." "Candles." "Matches."
Lina and Doon then fully understand that the river is the way out of Ember and must be traversed using the boat and paddles.
In chapter 19, after they follow the river in the boat, Lina and Doon find the entrance to a path on a rock shelf. On one of the rocks, Doon finds another very important and encouraging sign:
Welcome, Refugees from Ember!
This is the final stage of your journey.
Be prepared for a climb
that will take several hours.
Fill your bottles with water from the river.
We wish you good fortune,
The Builders. (Ch. 19)
Other signs mentioned throughout the book are the date sign that hangs in Harken Square, mentioned in chapter 2; signs in store windows that, as Lina observes in chapter 11, read "'Closed' or 'Open Mon. Tues. Only'"; and signs protestors are using to picket the mayor, which read, "WHAT solutions, Mayor Cole," as mentioned in chapter 12.