Li Tianji's - Skill of Xingyiquan is perhaps one of the best sources on Xing Yi around. It's not only well written but well illustrated as well.
Andrea Falk's website is
www.thewushucentre.com and the book ordering information, including a sample chapter and table of contents can be found at:
http://www.thewushucentre.com/litianji.htm
The book includes fundamental stances, strikes, and kicks, the five elements, 12 animals, linking form, mixture of moves form, some two-man, and qigong.
The sample chapter on the site is Chapter 3 which is the Linking of the 5 Phases (Elements) form also known as the Five Phases Connected Form.