Those books are great.
Check out the astral travel thread for more book recommendations 
Not yet, only a few times. But I will pick this up soon again with Captainās new Astral Viber field 
I donāt know the answer to your other questions.
I also donāt have much insight of what comes āaboveā the astral planes.
Robert A. Monroe in his books described the highest astral plane as āMonroe Level 35ā.
The āportalā to the level beyond āLevel 35ā was guarded by some powerful beings who told him that he cannot go higher until he reunites back with this Higher Self and then passes through the gate as a whole Higher Self with all its soul fragments integrated again.