Leo and Daisy books use the concept of "sight words" or "high frequency" words. Many words in the text are based on common words known as "sight words", DolchWords, or high-frequency words. These are very common words that a reader should recognize by sight and not have to sound out. The following web site has more in-depth discussions.
http://literacyconnections.com/

Currently, these books are targeted at certain Lexile reading score ranges. Each book is scored through the The Lexile Framework® for Reading Analyzer and printed on the back of each book. This is a great resource and I recommend their website.
www.Lexile.com

Please take time to research these topics. In no way am I connected with these web sites. I am not a representative for these sites and receive no compensation from them.