Journey of Souls, by Michael Newton is my favorite source. You can find an auditory copy on youtube. Delores Cannon, Life Between Lives, echoes Newton and expounds... she has many more books, and you can see her speak on youtube. She's no longer on the planet.

Robert Monroe, Journey of Souls, Thomas Campbell, My Big Theory of Everything, William Bullham... pretty much everyone that's been to the Monroe Institute, and Ingo Swann... They, too, have hit on this.