All three books of Monroe are great. His talk at the TMI from 1990 is also recommendable.
https://www.youtube.com/watch?v=GSBM7Qeo5bg (Part 1)
I also recommend William Buhlman's books, especially the first one called 'Adventures Out Of The Body'. It helped me learn and induce my first conscious and wilful OBE.
https://astralinfo.org/
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