Most likely: Journeys Out Of The Body, and/or Far Journeys, both by Robert Monroe. I recall borrowing both from the library and reading them when I was in my l

Most likely: Journeys Out Of The Body, and/or Far Journeys, both by Robert Monroe. I recall borrowing both from the library and reading them when I was in my late teens or early twenties, found them utterly fascinating. The Monroe Institute also had (or developed?) an audio technique to help entrain the brain, called Hemi-Sync - with a set of cassettes for OBE / lucid dream training, and various other cassettes to train other brain skills. In part they used a tech called 'Binaural Beats' (along with guided meditation), which is where one frequency, e.g. 440hz is played into one ear, and e.g. 450hz is played into the other, which makes one hear/percieve an internal oscillation of e.g. 10hz (the difference between them). The tapes used different frequencies to target different states, at different stages of the meditation, believing them to enhance different types of brain waves (e.g. 10hz is in the range for alpha waves), which was intended to teach the student how to invoke these states themselves at will. I still have some of his original cassettes, although nowadays they can be bought on CD IIRC (or perhaps snagged in mp3 format, if one looks around torrent sites). But I believe there is also an app nowadays. Interesting topic - and Monroe was an interesting guy. FWIW: back then, I believed in all manner of strange things, including OBE, but nowadays I'm mostly a rational skeptic. Monroe's books were very interesting to me back then, because they were quite fantastical, and claimed to have been written over many years of experiences.