What does the letter in book city of ember say?

You're likely referring to the book "City of Ember" by Jeanne DuPrau. In the book, the letter found by Lina and Doon contains a message from the city's founder, who describes the city's origins and the importance of finding the light.

Here's a summary of the letter's key points:

* The city's purpose: The city of Ember was built as a refuge for humanity after a devastating event.

* The light: The founder reveals that the city was designed with a light source that would guide them, but this light has been failing.

* Finding the light: The letter instructs future generations to find the source of the light and "rekindle the flame." This is the key to the city's survival.

The exact wording of the letter is not shared in the book, but the general message is clearly stated.

Let me know if you'd like more details about the book or have any other questions.