The meatiest technical book out there is “Astral Dynamics” by Robert Bruce. Here’s the sort of content you’ll find in it:
Despite containing little in the way of technique, Robert Monroe’s books are also pretty much essential reading to show how far you can take projection. “Journeys out of the Body” isn’t that special nowadays, but “Far Journeys” and “Ultimate Journey” take you way off the map…highly recommended! If you hoover these up and still want more, Bruce Moen series of books is an excellent way to continue.