This is the first book I've read concerning the idea that we all have more than just our physical body - that there are at least 2 others, the astral & etheric bodies.
In "The Mystery of the Human Double", Mr. Shirley discusses the "evidence" for these additional bodies, as well as the questions that are still in need of being answered (of which there are many since many scientists tend to shy away from such topics).
Many amazing examples are given re: people who have left their physical bodies and have then been seen & recognized by others. A variety of other types of examples are included, such as people who can describe a place they've never been in "physically", but have been in during some sort of "astral travel".
Overall, I found this book to be both easy to read, and extremely interesting. I would highly recommend it to those who are interested in the "paranormal" in general, and "the human double" &/or astral travel in particular. I found that it shed a lot of light on the topic.