Sepharial was the pen name of Walter Gorn Old, a British author who published books about astrology, the kabala, numerology, and other arcane topics in the early part of the 20th century. Here I am offering two of his books on crystal divination: Second Sight (1912) and How to Read the Crystal (1922).
Both these books are exceedingly brief--they could never be described as in-depth or incisive studies. However, they are both well-written and full of common sense (even if Sepharial's "scientific" explanation as to why crystal divination actually works is not very persuasive). Our author is wise enough to continually stress the spiritual nature of the practice, and he correctly condemns the use of magical techniques. He is very much aware that learning how to work with a crystal takes both patience and discipline. His advice about how we can do it ourselves strikes me as being very sound. I have never had much interest in working a crystal, but if I would start with the information in these two books if I ever decide to give it a try.
It is interesting to note that what Sepharial says about crystal gazing corresponds to what other writers on the subject have also had to say, including Theodore Besterman's Crystal Gazing (1924) and Northcote Whitridge Thomas's Crystal Gazing, Its History and Practice: With a Discussion of the Evidence for Telepathic Scrying (1905).
One final note: the astrological information at the end of the How to Read the Crystal doesn't have anything to do with crystal divination--it was probably tacked on because Sepharial's publisher wanted to add some filler to an already brief book.