Just came out and features brief interviews with tons of secular and Christian artists about the spirituality of music. Some transcripts of the interviews available here:
http://www.xtrememusic.org/metal.html

There is an interview with my band TEMPLE OF BLOOD in the Christian edition of the book:
http://www.xtrememusic.org/christian.html

Lots of interesting artists, but I would have prefered a bit longer, in depth interviews.

Is this just an online thing or actually a book?

It's a book (actually two books).

Ok thanks.