She starts with a brief recap of the relevant Freud, then goes into a detailed historical analysis of women's role through the centuries, all supporting her underlying premise, which is pretty much that women won't ever achieve social equality until she is producing something which both she and society see as worthwhile, *outside* of the domestic economy (which is so easily viewed as supplementary).
This book was written in 1949, but it remains incredibly relevant today; so many of the women (and men) I know in heterosexual couples are struggling with these questions of who works where, and how much. I'm not saying I completely agree with de Beauvoir (it seems evident that at least one other solution is to develop a society in which the domestic economy (child-rearing, etc.) is more highly valued, and that countries like Sweden are much further inclined in that direction than America is), but whether you end up agreeing with her or not, she's very smart and thought-provoking.
I'm going to buy another copy and send it to my younger sister. Maybe it'll help her stick it out through med school. :-)