Two great free online books from the Project Gutenberg.

Riding for Ladies – 1891

http://www.gutenberg.org/files/39610/39610-h/39610-h.htm#IV Complete with fetching illustrations of appropriate habits.

Ladies on Horseback, 1881  This one includes chapters on hunting:

http://www.gutenberg.org/files/39501/39501-h/39501-h.htm