The low blood sugar/coffee thing was recently studied and it was found that it does happen, but only in people who are not habituated to it.

As far as caffeine and dehydration, caffeine makes you pee slightly more than normal, like 1T more. Not exactly an amount that will lead to dehydration.

Are you talking about Dr. Dean Edell's book? He's not a nutritionist, he's was an opthomologist but then he got into talk radio. He likes to go after the medical urban legends and find the truth of things. Although, I think he's a little hung up on low carb diets (i.e. he doesn't like 'em).