What is the theme in City of Glass by Paul Auster?

One of the most prevalent themes of the book is that of what can be called manipulation. The main characters are manipulated into a situation in which they become trapped and are somewhat helpless to escape from. In City of Glass, Daniel Quinn can't walk away from the Stillman case. By the time he does, it is too late and he becomes the subject of the experiment itself. In Ghosts, Blue doesn't walk away from his assignment of following Black.