The book is useful - I use it to help me make scenarios. However, even though it is an 2000 version some of the information in it is far from up to date.
BTW:
I noticed the manpower figure too. IMO this is an error - Americans are not that unfit.
Maybe that has something to do with Conscription and Standing Army.
Maybe this could be implemented in Civ3 as well.
You got to chose between these two modes. Conscription however takes 5-10 turns to become effective.
Conscription:
Newly produced units start out with better experience, though no experience is earned when they stay in barracks...
A little less support costs.
Less moral.
Standing army:
Newly produced units start out with less experience though they get experience when they stay in barracks.
More support costs.
More moral.