Amazon has a new program where a reader can read as many books as they like for $9.99 a month. Be careful when "buying" one, as they have to be in the unlimited program.
This will allow me to exhaust all the available books on a topic without spending much money.
Currently, I'm reading all the books on Walking Across America. Then I'll move to the Appalachian Trail and other trails.
For those who prefer audio books, the Kindle Fire and the Kindle reader with the keyboard will read books aloud. I like to listen to books while I road walk, so this is a good deal for me.