Every great book has as many interpretations as readers.
Fight Club is a great book.
And I'm one of its multiple readers.
After enjoying the original version of the book, the Spanish translation and part of the Polish one (Podziemny Krąg - The Underground Circle), having watched the movie several times and of course after reading the rest of Palahniuk's works till today (except Invisible monsters, because I've never found it, and when I did I was broke) and, specially, after a night of literary discussion and beers with a friend, I came to the point in which I think it could be interesting to share my views on the book in two different ways, with even a third interpretation.
The first way is, of course, the one that most people keep. This is the view on the book everyone will agree on after watching the movie and thinking the whole thing is about a bored, white-collar worker who one day develops a severe mental dissorder and... If you haven't read the book, go find it and read it! and if you have, you already know the rest.
This Version A is interesting, in my opinion, because critics usually tend to keep the author's life and work in consideration before speaking their minds. And by critics I mean everyone who wants to make a good comment and critic on the book (there are as many critics as readers). So, in this case, they usually agree on one thing: the book is about a man who hates the world the way it is, he wants to burn it to ashes so he can see it resurrect stronger and he wants to destroy a sick culture and get rid of an even sicker cult - religion. Give it the name you want.
In order to do that, after failing to help himself to overcome his major problems (not sleeping, loneliness), he develops two concepts (his alter ego, Tyler Durden and Fight Club) to overcome the source of his problems: Marla Singer and Society.
Everything was fine after "Jack" (let's call him Jack, since everyone does it every single time anyway) could cry and sleep again. His problem, caused by a wicked world, was apparently overcome and he could continue with his empty life in a fucked up civilization. But then, Marla Singer.
Tyler Shows up when Jack can't overcome the existence of Marla, and he's clear about her in moments like: "get rid of her", "a generation of men raised by women. I'm wondering if another woman is the answer we really need" or simply by the way in which he wants Marla dead by the end of the story. Tyler is not only there to help Fight Club and Project Mayhem grow, but he's mainly there to keep Marla away from Jack (the way in which he achieves it is by having sex with her as "Tyler Durden", letting her think she is with Jack instead and making Jack think she isn't trying anything with him). This plan works and it goes on until the end of the movie, when Jack chooses Marla over Tyler and she chooses him, there's the shooting and then he's taken to a psychiatric hospital (the ending of the book is better, in my opinion).
So Version A means a story where a man creates an underground group to destroy the wicked world he lives in, trying to get himself away from this sick civilization that torments him.
Version B is the opposite. This is the version you interpret after reading every other book by Palahniuk. Why? Because Palahniuk writes about hurt people who may hate the world, they may want to fit in or they may want the world to recognise them... isn't that Jack?
Jack is a straight, single man with an empty life, who is bored of a routine empty of meaning. He needs a change, and maybe this change means changing the world, but the change he wants is smaller and... kinky: Marla Singer.
Marla is the complement to Jack. She has no control on things, she wears messy, second-hand clothes, she steals, she smokes (not that this really matters, but Jack doesn't). Tyler would consider her as the opposite, but she's, in fact, the complement to Jack's spice-less life.
One could think that Jack wants to change the world, but he doesn't. He's selfish and superficial: he wants furniture, clothes, to keep his appearance, money... what he lacks is a real woman to complement his persona, and all he wants is to complete his own pyramid of needs: a nest, security, food, self-realization and a woman.
He finds it in Marla. That's all he wanted, even if he looks angry at her, becayse he's just a little boy in the body of a white-collar worker and he has no clue about how to treat a woman.
But Marla would have meant a second self-deception, being the first one crying in order to sleep instead of fighting his real problems, and now Marla (a second level of social acceptance). Here is when Tyler shows up; he wants Jack to stop and face his real problems, his ghost: growing up and face the world out there. But he keeps trying to hold on Marla like a little kid on his mother's skirt.
Version B means a story of a lost man trying to fit into a society he can't understand, looking for love, finding it and overcoming all the problems, barriers and limitations he finds (Tyler Durden, Fight Club, Project Mayhem) to end up in peace with himself.
Now for the mindfuck! Since when is a woman who smokes allowed and accepted in a meeting of men with testicular cancer? This always looked to me like a joke by Palahniuk, but now I see it more like a sign for the third interpretation, Version C: Marla Singer, as much as Tyler Durden, is a product of Jack's mental condition. He's a lonely man who creates the perfect woman to fullfill the wicked needs that a wicked world made him have. Tyler shows up as the voice of "reason". But fuck reason! He's nuts, he wants to destroy the world, he starts achieving it and in his madness he gets caught and ends up in a mental institution.
I think this third interpretation is the one that fits the most. For me, that's the book.
But what makes this book a great one and a classic is the fact that you can find two other good interpretations and discuss them long and in detail.