When it becomes a piece of artwork... Have a look at this:
And then, there are the artists who make books - the 'artist book' is a genre all of its own. They can be made by artists who traditionally work on paper, or not. I've seen artist books made by ceramic artists - in clay (you can actually make a clay body incorporating paper pulp - it's light and strong, and offers a flexibility for sculptural pieces that can be very useful) - and all sorts of other mediums. The basic criteria is that the work needs to be in, or derived from, book form. While I was looking for examples I found this beautiful work by Gabriella Solti. You can see more of her work by going to her site.
But, artist though I am, mixed media artist at that, I can tear up all sorts of things to incorporate into artworks, but I can't tear up a book. Where I've used pages of text in odd pieces, I've made copies and used those pages instead of the originals... There's something, for me at least, something almost - and I use this word with some degree of caution - sacred about a book. This does, of course, fly in the face of any number of conventions, not least my own - not worshipping idols...! Maybe it's something to do with the notion of destroying ideas. Think back to when the Nazis burned 'un-German' books, and the quote that was used at the time, "They that start by burning books will end by burning men." Heinrich Heine (1797-1856), from his play Almansor (1821).
As I said in an earlier post when I was playing with ideas around what books actually are, books can be many different things to different people. For the artist, they can be truly many things - from a resource to becoming, literally, material for further creative endeavours. We artists are scavengers - anything is fair game. I still don't think I could destroy a book to make something else out of it...!