It is any book, old or new that has been recycled by creative   means into a work of art.  They can be ... rebound, painted, cut, burned, folded, added to, collaged in, gold-leafed, rubberstamped, drilled or otherwise adorned ...and yes it is legal!

(Courtesy of the ISABA - International Society of Altered Book Artists)

http://www.alteredbookartists.com/

Create

Evolve

Enigma of Time

The Letters

A Place For Me

Listen To The Whispers

Tree of Innocence