James H. Leuba 
Psychology of Religious Mysticism [PDF ebook] 

First Published in 1999. This is Volume III of six of a series on the Psychology of Religion. Written around 1925, this book is a psychological study of human nature. It includes a philosophical chapter and also one in which are set forth the practical consequences to religion of some of its conclusions. But, whatever may be the importance of these two chapters, the book is to be judged primarily as a psychological study of aspects of human nature conspicuous in mystical religion.