I read a lot of books about fasting during my fast. If you're interested, here are a few of them and a couple of thoughts on each.
Baab, Lynne M. Fasting: Spiritual Freedom Beyond Our Appetites. Downer’s Grove, IL, InterVarsity Press 2006.
An interesting book with a broad variety of good information… Ms. Baab walks thought a Christian history of fasting, medical issues in fasting, and fasting as a way to get freedom and deeper connection with God. Her definition of fasting is not related to food only, but in giving up anything for a limited time for a spiritual purpose.
Wimmer, Joseph F. Fasting in the New Testament. Ramsey, NJ, Paulist Press. 1982. This book had some really interesting insights into a variety of new testament passages, especially the 40 days in the desert for Christ. Mr. Wimmer spent a good amount of time focused on discovering the “original” text of scripture (i.e. the Q document and so on), so might be a little much for people who put a higher theological weight on the idea of the Bible being completely authentic (for instance, one of his big questions is, “Did Jesus actually historically go out into the desert and do a 40 day fast or is that just a story told for teaching purposes?”) I didn't walk away on board with everything the man thinks or even liking some of his questions, but he made some surprising connections and definitely provided "food for thought" ha ha ha ha ha ha. Pun intended.
Towns, Elmer L. Fasting for Spiritual Breakthrough. Ventura, CA, Regal Books, 1996. Essentially, Towns sees fasting as a tool to break various spiritual issues in life, and lists nine different Biblical examples of this and what he sees each “type” of fast entailing. I didn't find the book particularly convincing, honestly, because it doesn't match my experience or what I understand to be happening in the scriptures he looks at. It's a popular book and a popular point of view, though, and may be worth your time.
Bright, Bill. Your Personal Guide to Fasting and Prayer. This entire booklet is available at the link on the title. This is probably the most practical, brief and straightforward of the fasting books I read. It's designed to help people who are actually fasting, rather than learning about the theology or history of fasting. And it's free, and written by someone who did a lot of fasting, not just study about fasting.
Piper, John. A Hunger for God. Wheaton, IL. 1997. Crossway Books. A good, Christ-centered book on the power of fasting to help us reawaken and refocus ourselves toward God. It shows how fasting is a spiritual tool that allows us to desire God more in our lives. And, coincedentally, it's the inspiration for my own title on my own never-to-be-published book on fasting, Hungry For God and Cheeseburgers.
Foster, Richard. Celebration of Discipline. Only one chapter of this discussion of spiritual disciplines is actually about fasting, but it's a great, compact discussion of the potential positives in fasting and how to use fasting as a tool to grow closer to Christ.
McKnight, Scot. Fasting: The Ancient Practices. McKnight's book is probably my favorite new read during this fast. He focuses on fasting as a response to sacred moments in life, not as a tool to "get" anything from God. So, it might be that fasting comes about as a result of overwhelming grief or because one experiences God's presence in an overwhelming way (or desires to) but not because it moves God in any way. In fact, he argues that fasting is our "natural" response to sacred moments and it's not something we need to practice or prepare for, but will be our natural response in certain situations. I'm simplifying and no doubt doing violence to his argument, but this is one I would gladly recommend.