I'd recommend reading both. Bert Herman's book predates Deborah Addington's, and has quite a different focus. Disclaimer: I have minor professional ties with both authors under my Mistress Lorelei pen name. (http://mistressmanual.com/)

Also, you should check out this resource for (fundamentalist) Christians: Fisting and God's Will. (http://www.sexinchrist.com/fist.html) The author is right that fisting is a profoundly spiritual experience for both giver and receiver.