The first book I'd recommend to anyone who has an interest in prayer, spirituality, religion, healing; and the relationship of those to the replicable, evidence-based pursuits of science and medicine. Not flawless, but undeniably thought-provoking. (I say that as a former religious zealot who spent many years as an atheist, agnostic, and now non-dogmatic believer in something.) The author is a medical doctor of internal medicine.
Topics covered include locality—prayer doesn't need to go anywhere or to anyone; the ethics of prayer in medicine; how to pray; how studying prayer may differ from studying other subjects; and of course disentangling prayer from placebo effects.
I thought the end of the book drifted into religious platitudes, but overall I loved reading this. It's accessible to a general audience and also has citations in the endnotes.