There are plenty of books about astral projection out there, but these are the ones I personally recommend.
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(Click the title to see more information on each book. These are affiliate links, which I have provided for your convenience, but it does mean that I will receive a small commission if you happen to buy any of them. This comes at no extra cost to you or anything. Some of these books may be available for free at your local library or on Kindle Unlimited or thru Audible.)
Journeys Out of the Body: The Classic Work on Out-of-Body Experience by Robert A. Monroe
This is part of a trilogy, and I've included the other two books as well:
Adventures in Consciousness by Jane Roberts
Seth, Dreams and Projections of Consciousness by Jane Roberts
I always include Seth books on this list. Even though they are not exclusively written on the subject on astral projection, they do mention it (usually using different terms) and they discuss related subjects.
How to Develop Your ESP Power: The First Published Encounter with Seth by Jane Roberts
The Seth Material by Jane Roberts
Seth Speaks: The Eternal Validity of the Soul by Jane Roberts
I haven't personally read the following book, yet, but I've heard good things. Since it's on my list, I figured I'd include it here.
Mastering Astral Projection: 90-Day Guide to Out-of-Body-Experience by Robert Bruce and Brian Mercer