Alban, I don't know whether or not I've already said this on the forum, but in any case it can bear being said again: Anyone interested in photography as an art form MUST find and read Looking In, the catalog for Robert Frank's current show at the Metropolitan. If you possibly can, get the hardcover "expanded" edition which includes the contact sheets from which the pictures in Frank's The Americans were selected. You can learn a lot more about photography by looking at a master photographer's contacts than you can by looking at his finished product.
While you're looking, the best place to start might be Eugene Atget, and then on to Cartier-Bresson. Don't overlook anything by Garry Winogrand or Lee Friedlander, and, above all, Walker Evans. As Mike pointed out there are many, many more recommendations on this forum, so I'll stop here with the suggestions.
One thing's for sure: no amount of reading how-to books or articles about equipment is going to improve your photography. As Cartier-Bresson said, "Photographing is nothing. Looking is everything." Learning to look is what studying the work of our greatest predecessors is all about.