JReadability is a utility that generates a reading grade level for a given text (download details here). There is a index for 5000 books sorted first by Obi-2 and then Hayashi. A paper describing the index is available here as a pdf.
In Obi-2 scores the higher the number the more difficult the text is to read. The number relates to the equivalent grade level of the Japanese schooling system, with 13 considered as university level. In contrast in the Hayashi index higher numbers mean the text is easier to read. An Obi-2 indexing tool is available online on this page.