He and Rooney actually wrote the original 40yd dash book... I am not sure if it is revised since then. Roon is Bill Parisi's director of training from what I can tell.
The book is a good buy if getting better at the 40 is a huge goal. Remember, the 40 is a test...like the SAT... does the SAT tell you that you are smart? Nope... it tells you that you can perform the SAT...
Now, explosive lower body power is associated with the 40, but even more so with the vertical as far as I am concerned..... well let's just say the two together are great.
So, is it a great book? Yes, for sure.
If you want to get good at the 40....
His combine video is great also...as are all of the Parisi videos.