The book of Gelfand and Fomin is a good place to start. It worked for me. I would like to include another nice and short source namely Chapter 19, vol. II of *Feynman's Lectures on Physics*. If you know a little about smooth manifolds, then Arnolds's *Mathematical Methods of Classical Mechanics* is another excellent source. Also, check volume in of Dubrovin, Fomenko, Bovikov, *Modern Geometry*.