I have had a copy of this book since the early 1970s. It was published in 1960, thus avoids the 1960s New Age Movement and the wave of innovations that issued voluminous writings by spiritual and religious teachers. These writings eclipsed all previous periods Gaskell helps researcchers establish a baseline to survey concepts through the nineteenth century and into the 1950s.
I strongly recommend this as an essential reference for scholars and anyone interested in comparative spirituality. It also provides a useful complement to Manly P Hall’s survey of Freemasonry and related teachings in his The Secret Teachings of All Ages. Other useful surveys may be found in publications of the Golden Dawn and the Philosophi8cal Society. Four Stars for preserving key citations marking a significant transition point in the history of sacred writings. I might also recommend the site established by the late John Bruno Hare at sacred-texts.org/
I think this book is a lot more helpful to understand theologic structures of teachings and religions than the books of madam Blavatsky. Because unless you have a good grasp of the eastern and western religions &mythologies, you would be choked of many unknown terminologic words. However, in this book there is no such a problem for the first timer students.