"Fight Club" is about life in the Western world at the end of the twentieth
century. More specifically it's about what's happening to men in America.
The two main characters the nameless narrator, which I'll refer to as "Jack",
and Tyler Durden each represent one side of the increasingly divided American 
man. The narrator represents dull boring conventionality. Tyler on the other
hand is a man of action.

Jack is clever. Jack thinks too much. All work and no play make Jack a dull
boy. Jack feels sorry for himself. Jack is weak. This is how Tyler sees Jack.
Or rather how Jack idea of what Tyler thinks of him.

Jack's life is empty. He has many problems that he chooses to ignore. This
is probably what leads to his insomnia, a symptom of a greater disease. He
fills the void in his life by "losing himself" in the support groups. Fight
Club is the result of applying the idea of losing oneself to all aspects
of one's life. This is a recurring theme in the novel.

Jack loses time and gains time flying around the country. He loses himself
in the support groups. He is losing himself to Tyler who is taking over his
mind.

Tyler on the other hand is in control. However on closer inspection one finds
that he is not going anywhere Jack would want to go. In fact after the initial
excitement of fight club wears off, Jack is left in a pretty sad state. He
does not approve of the mayhem and destruction. He even tries to stop Project
Mayhem, which very nearly costs him his balls.

Tyler is not a leader of men. He leads space monkeys. At best he is the leader
of a successful cult. He is a modern day Peter Pan, he refers to his generation
as one of boys who have never grown up, the difference being that he leads
his followers to attack a larger foe: modern society itself.

Jack and Tyler represent the two extreme options available to American men.
There is only one Tyler, one leader. There are many Jack's all fearing, all
believing. In this way the relationship between Tyler and Jack is very similar
to that between the people and "their" government, or at least what the author
thinks the government is headed towards.

However the important idea that the author is getting across is that no one
wants to think of them-self as a mindless follower. This is why Jack invents
Tyler. This is what causes Jack to "lose it". This is the mark of a psychology
of strength. When one respects only strength one is forced to submit to those
who are stronger and to bully those who are weaker. This is why Jack a modern
man, the weakest of the weak, is forced to terrorise himself.