This represents a large category with phenomena that some would prefer to keep separate. The existence of ghosts has been controversial for centuries with no definitive evidence either way. Certainly many of the famous stories of ghosts have perfectly reasonable alternative explanations. Keep in mind that a belief in life after death does not require a belief in ghosts and that a belief in ghosts and other spirits does not logically entail a belief that mediums, spiritualists, and channelers can make contact with them or any other spiritual essence.
Blackmore, Susan. Dying to Live: Near-Death Experiences. Prometheus Books, 1993. A psychologist who has spent much of her professional life in sympathetic attempts to explore claims of parapsychology looks at the psychology and biology of near-death experiences. A good review of the various literatures on this topic.
Edwards, Paul. Reincarnation: A Critical Examination. Prometheus, 1996. Critical look at alleged case studies and analysis of some of the logical and philosophical problems involved with the concept.
Finucane, R.C. Ghosts: Appearances of the Dead & Cultural Transformation. Prometheus Books, 1996. An historical treatment of how ghosts have been construed throughout history.
Gordon, Henry. Channeling into a New Age: The "Teachings" of Shirley McLaine and Other Such Gurus. Prometheus, 1988. A critical look at channeling and other New Age beliefs about spiritual matters.
Lewis, James R. Encyclopedia of Death and the Afterlife (*) Visible Ink. A straightforward treatment. Particularly strong on non-traditional religions and their beliefs about the after-life.
Moody, Raymond. Life After Life.(*) Bantam Books, 1975. The book that began the near-death experience movement. The author carefully refrains from pushing the idea that such experiences constitute contact with an after-life, but he does see them as having a strong spiritual component.
Randles, Jenny & Hough, Peter. The Afterlife: An Investigation into the Mysteries of Life After Death (*). Berkley Books, 1993. Despite book cover claims to the contrary this is hardly an unbiased treatment, but it does provide a readable albeit somewhat credulous summary of paranormal phenomena associated with the after-life such as after-death experiences, ghosts, hauntings, and séances.
Ring, Kenneth. Heading Toward Omega: In Search of the Meaning of the Near-Death Experience. William Morrow, 1984. This and the his earlier, Life After Death: A Scientific Investigation of the Near-Death Experience (Coward, McCann, & Geoghegan, 1980), present the best data we presently have on the prevalence and nature of near-death experiences. There is no doubt that people have such experiences; the issue is how they should be interpreted and whether they provide convincing evidence for a life after death. Ring's earlier book focuses on data about the actual experiences, while the latter is more concerned with how such experiences affect those who have them. Ring presents some scientific explanations but is a clear believer that such experiences constitute a contact with a spiritual world (although one that does not fit conventional Christian notions). A more recent book, The Omega Project: Near Death Experiences, UFO Encounters, and Mind at Large (Morrow, 1992), attempts to trace the parallels between near death experiences and reports of people who claim to have been abducted by aliens. Although tastes are bound to vary in such matters, to me the parallels seem superficial, and there is no a priori reason to believe they are similar.
Tanner, Amy. Studies in Spiritualism (*). Prometheus Books (reprinted). This is a classic study of Mrs. Leonora Piper , a famous spiritualist and medium around the turn of the century. Tanner recorded several séances that she and G. Stanley Hall, a famous early American psychologist, attended, and they concluded that Mrs. Piper probably did suffer from various disassociative processes (perhaps multiple personalities) but that she also used deception.