I would say that if you want a good basic primer on the subject, get Baldwin's book. But there are other books that fill in the gaps and also correct Baldwin's errors.
"Thirty Years Among the Dead" by Carl Wickland is useful and, I believe, you can get the online transcript of this book here:
http://www.harvestfields.ca/ebook/01/063/00.htmThen, there is the work of
Edith Fiore, and
Joel Whitton, a psychologist and psychiatrist respectively. Fiore wrote
"The Unquiet Dead" and you can get it cheap from amazon:
http://www.amazon.com/gp/product/0345350839/103-8048209-6367806?v=glance&n=283155Whitton's book,
"Life Between Life" can also be obtained inexpensively:
http://www.amazon.com/gp/product/0446347620/103-8048209-6367806?v=glance&n=283155Allen Kardec is another useful source:
http://www.allan-kardec.info/And certainly, you might want to read
Malachi Martin's work on possession:
"Hostage to the Devil", for a good idea of the differences between just ordinary "attachment" issues and the full bore possession problem. Naturally, I don't have much confidence in the Catholic rite of exorcism, since I am convinced from my own experience that the success of the exorcism has more to do with the exorcist than the rite, but it is still a very useful book and lays out some valid principles.
Nandor Fodor's book:
"The Haunted Mind- A Psychoanalyst Looks at the Supernatural" is excellent.
A good general knowledge of psychology is also very helpful. It strikes me that what is often called "dissociation" can be a key to what I previously referred to as "thought energy globs." At the same time, one cannot discount simple dissociation as what it is... a compartmentalization of the mind, or the manifestation of the many *I*s discussed by Mouravieff and Gurdjeiff, of the "predator's mind" of Castaneda. So, do read
Martha Stout's book
"The Myth of Sanity." Then, of course, one must consider past life memory and how it can create issues in the body (and mind and life experiences) that may seem almost identical to the apparent symptoms of "attachment." This is where body work really comes in. I worked very closely with a great therapist in Florida, Terry Ames, who had numerous experiences with his patients where working on physical problems would induce spontaneous past life recall. He developed a technique something like psychotherapy to "talk them through" it while he was working on the injury or trauma in the body. He had some amazing successes. In other words, you can be "attached" by (or attached to) your own subconscious memories of the past, prior traumas, etc.
The bottom line is: it is a very complex subject and Baldwin gives it a pretty good background, but he got stuck in belief systems and went off track on a couple of areas where he just simply didn't have either the experience or the knowledge to develop those areas adequately.
A very good "beginner's" source to work with some of these concepts is
Dick Sutphen's videos. He's gone a bit too new agey for my tastes in the past 10 years or so, (probably because he married that flakey woman "Tara" and got vectored by her) but he still has produced some excellent tapes and videos for "home use." His site is here:
http://www.dicksutphen.com/ and you can see his selection of tapes and videos here:
http://www.prohypnosis.com/ Just don't waste any money on anything by Tara Sutphen. One of his tapes that I found to be MOST useful was called "Beta to Theta." It is a recording of the "tick tocks" of something like a metronome that begins at a certain frequency (ordinary brain waves) and gets slower and slower until the beats are at the Theta frequency. There is no voice, no suggestions, nothing... which enables one who is experienced at achieving these states to do their own "work."
Hope this small list helps.