Books, Books, Books
I found quite a few books.
There is a Google Preview of a few books on the Navajo language. A few pages of these books are available to read online, but pages are intermittently unavailable for viewing. However, there is plenty of useful content that can be acquired from these previews. I would suggest reviewing these books before actually purchasing them to see if it would benefit your particular learning strategy. I’ve already been reading The Navajo Verb by Leonard Faltz and I have made a committment to purchase a copy.
Archive.org and Gutenberg.org have a few books available that are in Navajo. I found a bilingual social studies book, a dictionary, a short story and a few books with excerpts from the Bible. Very interesting stuff.
Note: These are books that are viewable online at no cost. In some cases they are available for download to print or read on your Kindle.
There is a separate page for books and programs for purchase here.