The Varieties of Religious Experience by William James

William James was a leading American philosopher and psychologist in the 19th century.  James was the first person to offer a psychology course in the United States and he is often referred to as the Father of American psychology. James's brother, Henry, was also a famous author.

The Varieties of Religious Experience, published in 1902, is considered by many to be James's greatest work.  The book is based off lectures James gave on natural theology.