Amazon.com has been carrying a Text Stats feature in the "Inside This Book" section of most book-detail pages since a long time that can help you gauge the readability level of books.
They are calculated based on the following rating techniques -
- The Fog Index was developed by Robert Gunning. It indicates the number of years of formal education required to read and understand a passage of text.
- The Flesch Index, developed in 1940 by Dr. Rudolph Flesch, is another indicator of reading ease. The score returned is based on a 100 point scale, with 100 being easiest to read. Scores between 90 and 100 are appropriate for 5th and 6th graders, while a college degree is considered necessary to understand text with a score between 0 and 30.
- The Flesch-Kincaid Index is a refinement to the Flesch Index that tries to relate the score to a U.S. grade level. For example, text with a Flesch-Kincaid score of 10.1 would be considered suitable for someone with a 10th grade or higher reading level.