HOW TO find readability level of search results & books

Google's “Instant Preview” lets you visually preview a search result page before you actually visit a page. A new advanced search filter called Readability level now lets you predetermine whether the result pages you want to see Basic, Intermediate or Advanced. I could not find details on what methodology Google uses to judge the readability level.

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.