I work with kids who are behind the curve in reading. One way for a kid to figure out if they should be reading a book is to read the first page, then try to count the words they don't know. If they know all the words, the book is probably beneath them, and if they count four or five words on the page they don't know, they probably won't be able to read the book at all. Two or three words is just right because it means the book is teaching new things to the kid, but not to the point where they can't read it.