Are there any free apps where I can listen to books while I am working out?

Try googling "free audio books" and check out some of the more credible looking sites. Also, if you haven't already, see if your local public library offers digital audiobooks. Even if they don't, the librarians may be able to help you find a good source for listening material.

I love audio books. I spent the winter walking -trying to run, listening to Harry Potter. A lot of the classics are available on Librivox and other sites.