The Accidental Savior

In Firewatch, you can pick up some Objects to interact with.

Summary
The book is found inside of Cache 307.

Behind the Scenes
The Accidental Savior was the book written by Terrence L. Greenbriar from Fullbright's Gone Home. It was included as a nod to Gone Home as it was one of the inspirations for Firewatch. Campo Santo and Fullbright have also said that this suggests the games take place in the same universe as Gone Home.