The first thing you should know about this book is that this Swami author isn't Indian. He may very well be a swami, possibly in the way that Bill Gates has a doctorate from Harvard, honorarily, but he isn't a native of the Indian continent. The author is, in truth, William Walker Atkinson. Wikipedia describes him as a lawyer from Baltimore who started his business career in 1882.
That bit of truthfulness out of the way, this book addresses the primary principles of clairvoyance. Some of the terminology and scientific references are obviously outdated, but he does a lot of useful explaining and presents material that is reinforced by other reading I have done on this subject. Parts of it read like a self affirmation pamphlet and much of the rest of it is presented in an unappealing “golly gee whiz” sort of way, but the general content is widely corroborated.
The most unfortunate part of the book is that it actively avoids any sort of scientific explanation of phenomena he cites. What he provides are the results of experiment data proving that telepathy works, but nothing explaining why or how. I counted ten instances where the author begged off scientific explanation as too complex to go into. Even if I didn't have the math, chemistry and science background to understand an explanation, I would've liked to have at least seen one.