Human Demography - Where can I find this statistic?
The CDC has some good stuff. http://www.cdc.gov/nchs/data/nhsr/nhsr051.pdf

But keep in mind that number from your book is likely based on yearly findings in which more than half the women would not be able to have children due to age, voluntary contraception, or other conditions. So it is not reflecting how many children they have over their lifetime. Even in the CDC study more than half the women had no children but few had no plans to have children, couples are moving their first pregnancy to later and later in life, which is going to make it look like few are having children at all on first glance. 
 And it's probably not accounting for death rates, the poorest people tend to have the most children but they also have the fewest number live to maturity, which is why they tend to have more (not necessarily consciously mind you) .