Not all books are free on kindle. Best sellers or recent books will usually have to be purchased from the Kindle store. There are however kindle books which can be found for free. These are books which are old enough that they have entered the public domain. These include books like Huckleberry Fin and Arabian Nights. They are available at places like Manybooks dot net.
Old classics in the public domain are available on Amazon for $1-5, but it is pretty easy to find free PDFs or word documents of them online, though they will need to be converted before being read on kindle since kindle does not support Microsoft word or PDF formats, though it does support .txt formats. Thus, not all kindle books are free, but it is typically easy to find free books that can then be converted to a kindle format. This is however only true of public domain books and not recent books or best-sellers.