Hello everybody, I would like to know about the work of Élie Cartan of PDE's that relate to the theory of foliations and differential forms.

I am interested in the subject and will be happy to receive basic references on the subject (articles) as well as explanations on the importance of the subject in mathematics today.

Exterior Differential Systems, which is available for free from the MSRI: msri.org/communications/books/Book18/… – José Figueroa-O'Farrill Aug 27 '10 at 16:13