The Lexile Framework for Reading is an educational tool that uses a measure called a Lexile to match readers with books, articles and other leveled reading resources. Here's a detailed article about this framework: https://en.wikipedia.org/wiki/Lexile.
You can level your books following Lexile's framework. This makes it easy for teachers (e.g., Common Core programs in the USA) to use your books in their classrooms.
Lexile's Analyzer and Wordlists tools can be very handy.
tags: reading levels, reading comprehension, lexile, books, common core