Reminds me of some of the parenting books I've been reading.... Studies have shown that there is (on average) no developmental benefit from early reading and a mild possibility of negative impact.
I went back and checked and unfortunately the book itself simply says "studies support the following general conclusions..." and don't give references (not exactly a scientific publication!).
Note that it's not in any way suggesting preventing children from learning to read if they want to and show an active interest, nor from stimulating them with an environment rich in reading-relevant material (games with letters, numbers etc.). It is simply saying that sitting them down and giving them lessons in how to read or anything else they are not developmentally ready for is probably going to be counterproductive.
What books and studies suggest this?