The following are books that I have used for tadoku (extensive reading) in Japanese. It is mostly focussed on book series, as I like to find a series and read through it to get as close to a "graded reader" experience as possible.
Japanese Graded Readers
I started using these before I really understood the rules of tadoku. Once I did learn the rules, I made a huge leap in progress. Because these books are graded, I won't go into detail about the content. As far as I know, these books are the only graded readers available in Japanese. Each volume includes a CD for listening practice.