As Myridon explained, we use the word "reader" to refer to a specific type of book designed to teach children how to read. (You might use the same type of book to teach an illiterate adult to read, but it would be a little odd, because the content is aimed at children.)
A book that is "an easy read" is a book that is not difficult to read, whatever the age group it's aimed at.
That is why we have asked you to tell us what you want your sentence to mean. It can't refer to both concepts at the same time.