This book deals less with spirit photography than with mediums & ectoplasm spewed forth from them. The pictures shown as well as the fact that so many mediums involved in that type of display were exposed as frauds makes one less apt to believe that these are actually spirits captured on film. Double exposures look more real in comparison. Nor does this book even show the few photographs of ghosts that have been tested & believed to be genuine (such as the hooded figures on the famous Tulip staircase, the brown lady of Raynham Hall, the faces trailing behind the S.S Watertown, etc...)
Another reason to doubt the fact that these are true entities is that these displays are no longer carried on by modern mediums.