I have heard that is a good book. From a personal trainers perspective, you want to work your "transverse" abs. These include your obliques, and deeper core muscles. Basically, the muscles that hold your abs in tight.
You can accomplish this by using rotation when you train your abs. Woodchoppers, Russian twists, bicycles and maybe some captain's chair exercises will do wonders for you!
You can search core exercises to tighten the whole abdominal region. Don't forget your lower abs.
Good luck!
PS At this point, just focusing on your rectus abdominals may even be counterproductive if you have separation. Think about working your abs without having the middle portion engaging first.