Here are a few references for differential forms.
Web sites
Books
- Harley Flanders, Differential forms with Applications to the Physical
Sciences (Dover, NY)
- Richard E. Williamson, Richard H. Crowell, and Hale F. Trotter, Calculus
of Vector functions, Prentice-Hall, 1968.
- Michael Spivak, Calculus on Manifolds, Benjamin, !965.