DuBay, William. Smart Language: Readers, readability, and the grading of text. Costa Mesa, CA.: Impact Information, 2007.

DuBay, William. Unlocking Language: The classic readability studies. Costa Mesa, CA.: Impact Information, 2007.

Evetts, Julian and Michel Gautier, Literacy Task Assessment Guide. Ottawa: Statistics Canada, 2005. Downloadable at: ibd.ab.ca/

Flesch, Rudolph. The Art of Readable Writing, 1949. Downloadable from:

http://dc135.files.wordpress.com/2012/11/flesch-the-art-of-readable-writing.pdf

https://readability-score.com/

An easy-to-use online readability calculator