Between XX and XY: Intersexuality and the Myth of Two Sexes Book

If you've never thought beyond the concept of "opposite sexes" this is a really great primer. If you're a gender and sexuality nerd like me, you won't learn a whole lot of brand new stuff from this book.

Callahan spends a lot of time debunking the myth of the two sexes - there's slightly more in the book about that than there is about disorders of sexual development or intersex. If you're looking for a really in-depth read that focuses on intersex, this isn't that book. But if you're looking for some more general stuff about the science of the differences between men and women and genetic variations that will tweak your concepts of "men" and "women", this book is very much worth your time.

Callahan was able to write a very readable book that didn't bore me or confuse me at all, even when he writes about chromosomes and karyotypes and stuff. In addition to good content about human sexuality, he devotes a good amount of time to writing about hermaphroditic animals, which is fascinating stuff and will further erode your notions of a two-sex model. The case studies of intersex people included in the book are also engaging, though also pretty heartbreaking.

If you are up for something heavier, more theory-laden and more advanced about the differences among the sexes (see what I did there? implying that there are more than two sexes with grammar?), check out Anne Fausto-Sterling's Myths of Gender: Biological Theories About Women and Men. That book was originally published in 1987, updated in 1992 - so some of the science is a little our of date, but it's a really great and challenging read.