The best one I've found so far is "Body Reflexology: Healing at Your Fingertips" by Mildred Carter and Tammy Weber. It also includes information on other therapies to support reflexology.
She's been practicing and writing about reflexology for decades, so she has plenty of experience at what she does. The best thing about her books in general are that they contain a fair number of pictures and diagrams to show you what you're doing.
However, I honestly didn't look very far and sometimes Mildred Carter's writing style gets on my nerves. She's still better than most of the older generation of alt.med writers.