During Don's UFO research he actively researched information on the negative polarities UFO elements. I wonder if this damaged his mind a little. Also, the problem of 'psychicly connecting' with such beings once it is clear who has reached the table.
I think this is the reason for his death. This is the thing that was able to be stimulated inside him in order to de- tune the contact with excess transcient questions.
I am of course not sure. But neg ET's. Truly terrifying area.
I don't know whether he could have returned under such circumstances, perhaps the connection to this whole area has come through the transcient questions in the earlier books.