College at 9?

So Carsyn has been asking me for a while to get him a human anatomy book. He LOVES to learn about how the body works. I've been shopping around and I had one all picked out for 5th grade and I figured I would get it as part of his curriculum next year. While at the DI today We ran across a book that was "Body Structures and Functions" So I tossed it in the cart. I get home he is SUPER excited to read it so it was his book of choice for Quiet time. Then he is telling me all about what he learned. So I did an internet search of the grade level for this book I found that it is a COLLEGE level book! I told him he is more then welcome to read and learn from it as long as he is comprehending it and doesn't get too frustrated. I then went through and found the reproductive section and will tape that shut lol.