Theory:

The title of the lesson "The Accidental Tourist" refers to the narrator himself. A tourist is someone who travels to places for fun and pleasure. But here in the lesson, the narrator calls himself an accidental tourist because, unlike the real tourist, travelling is a dreadful experience for the narrator.
  
A tourist is also a stranger to the places he travels to, and so is the narrator. To the narrator, the real world is as alien as the place that a tourist travels to. As the narrator says at the beginning of the lesson, 'of all the things that the narrator is not very good at, living in the real world is perhaps the most outstanding'.
 
Hence, through the lesson, the narrator talks about how he is an accidental tourist in the real world.
 
Important!
Note:
 
The lesson "The Accidental Tourist" by Bill Bryson should not be confused with the novel of the same name by Anne Tyler published in \(1985\).