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
