Maybe I'm wrong, and it's all true, but I suspect that the book might be a mix of things. Maybe Billy did communicate to some degree with Annie, which then she mixed with some channeling and/or thoughts of her own.
In any case, the book is a nice read and gives you some food for thought.
Thanks for the book review. I haven't read it myself, but by the sound of it, your interpretation above could be correct.
"Certain music written by the composer Sibelius [Finnish composer] gives you an idea of what the cosmic sound is like. Sibelius was definitely tuned to a higher dimension. I'm not talking about his darker pieces, but download his swan music and notice how the swells of sound break into melodies. This will give you a hint of what I hear, except what I hear is infinitely lighter and more sublime."
Kagan, Annie. The Afterlife of Billy Fingers: How My Bad-Boy Brother Proved to Me There's Life After Death (p. 133). Red Wheel Weiser. Kindle Edition.
Here's "The Swan of Tuonela" by Jean Sibelius. It certainly sounds "cosmic" and celestial. (There are many people in the comment section, who have found out about the composition from reading the book.)