The following is a great site which you can download to your computer.
The crafty site has lots of information so have fun exploring!
Beginning watercolor: http://www.craftsy.com/reference/world-of-watercolor?ext=NFB_Painting_Acquisition_Core_Free_Downloadable_LALxINT__wrldofWCNS#_=_
It's a blog so be sure you check out the other articles on the side. There is
a lot of information about composition there, also.
This site is an advertisement for a book. But there is a lot of information about Degas and his use of composition: