Introduction
I don't own very many books on astral projection and out of body experiences (OBE's), mostly because they quickly get into areas I'm not personally interested in at this time, including theories about planes of existence, Akashic Records, saving souls, etc. (I'll be more than happy to work on saving souls later, when I'm convinced all of this is real.)
As I mentioned in my About page, I'm more of a nuts and bolts person, and I just want to know if all of this is actually real, that this isn't all some sort of psychological problem. To that end, my desire is to get my "second body" (astral body, soul, energy body) out of the physical body as often as possible, but to spend my time hanging out here in the Physical Realm/Plane of Existence. (And this is probably the only time you'll read me writing about "planes of existence" or "realms".)
Recommendations
Given that introduction, here are several books on OBE's and astral projection that I do recommend:
As far as I am concerned, "Journeys Out of the Body" by Robert Monroe is the classic text in the field. Mr. Monroe writes about what happened to him, why he thinks it happened (his experience with sound recordings), and documents all the variables he thinks are related to having OBE's. If you can buy only one book, almost 40 years after its first publication, this is still the one to get.
"Astral Dynamics" by Robert Bruce came recommended by several friends, and it is good, but IMHO it needs some editing. It is over 500 pages long, and while the content is good, I believe it can be condensed significantly.
Finally, while "Remote Viewing Secrets" by Joseph McMoneagle is about Remote Viewing, and not OBE's, it's still a very interesting read because Mr. McMoneagle writes a lot about "protocol", and I think this is the sort of thing needed to research these events scientifically. If you're only interested in OBE's don't buy this book, but if you're interested in the larger field of psychic phenomena, this is a very nice read.