Out of curiosity, I put "The Elegance of Nature-Book I: Prophecies of the New World" to the test.
Flesch-Kincaid Reading Ease: 72.3
(A higher score indicates easier readability; scores usually range between 0 and 100.)
Flesch-Kincaid Grade Level: 7.6
Automated Readability Index: 8.2
A grade level (based on the USA education system) is equivalent to the number of years of education a person has had. A score of around 10-12 is roughly the reading level on completion of high school. Text to be read by the general public should aim for a grade level of around 8.
All in all - A very handy tool to have at our disposal!