What is an "altered book", basically it is an old book in which you create another piece of art. The altered book may have a theme to it, or it may not, it is entirely up to the creator. You can use all mediums inside your altered book, paint the pages, glue pages together, use collage, fabric anything you desire. I started my first altered book last year and am waiting for it to come back "home" as it went on a round robin - this is where there are several people who form a group and each of use create a page (or more) in each other's book.
By searching the internet you can find more information on this aspect of creativity, altering a book has also broadened with altering other objects such as CDs, tins etc. I certainly have found altering books fun!
Some of my favourite altered book places and resources are on the left.