You can only Booktrack work that you have created or have the rights to use. These would include any of your own original work and any royalty free text like Romeo and Juliet by William Shakespeare that's available for anyone to use.
For a sample royalty free short story you can use to create a Booktrack, click here.
For a sample royalty free poem you can use to create a Booktrack click here.
For an extensive list of royalty free books, stories, and poems that you can use to create a Booktrack, visit the Gutenberg Project.
If an Author has use a license that allows modifications or reuse, then you are likely to be able to use the work. It's always worth checking the details to be sure you are allowed to.