There has been a number of debates, ranging from psychologists to philosophers, from scientists to theologists, from shamans to neuroscienctists, over the concept of "consciousness" for a long, long time. But, the "consciousness" is usually defined as a state of awareness or perception of one's surroundings and one's own state of mind - of being conscious. Now, we have a different concept that is related: altered states of consciousness (or ASC, for short), which Tart has defined as "qualitative shift in his pattern of mental functioning, that is, he feels not just a quantitative shift (more or less alert, more or less visual imagery, sharper or duller, etc.), but also that some quality or qualities of his mental processes are different" (p. 1). This usually means that a person would experience a different perception in space-time or distortion of one's visual perception or dissolution of one's sense of self.
Now here we have a book, as edited by Charles Tart, that is a collection of essays, first published in 1969 and updated by 1990, that explores the concept of ASC and covers the effects of drugs, meditation, hypnosis, and dreams. These essays were written by a number of well-known figures, including William James, Arthur C. Hastings, Milton H. Erickson, Wolfgang Luthe, and even include four essays by Dr. Tart himself. This book, consisted of 650+ pages, goes on for thirty-five chapters with eight sections. These sections focus on a general discussion on ASC, hypnagogic state, dreams, meditation, hypnosis, psychedelic drugs (both minor and major), and the psycho-physiology of ASC. Most of these essays can be technical but interesting while others are less and more insightful.
Since this is considered to be out of date by a few decades, I still found this comprehensive survey to be quite a fascinating study on altered states of consciousness, and these pages were backed by credible sources and references to which I'd found myself to look into. Indeed, it is a long read but it doesn't have to be read from cover to cover. Choose one section or one chapter that interests you and learn something interesting about that subject. Very informative reading, in my opinion.