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/