Let's Talk About Sex tackles the subject in a matter-of-fact manner, including HIV, Aids and the ultimate taboo in much of the United States - homosexuality and lesbianism.
A former teacher, Ms Harris has found herself part of the battle between American fundamentalists and ordinary parents, 80 per cent of whom want their children to receive sex education at school.
What little uproar there was about the book - called It's Perfectly Normal in the US - was, she says, clearly orchestrated. "In Tacoma county, Washington state, there was a mother who fought to get the book out of the school library, but she didn't even have a child in the school.
"It's very intimidating. It is the librarians in our country who are the real heroes. In a democracy, all points of view need to be expressed and we need to bring up children to have a diversity of ideas and thoughts."
Ms Harris, from Cambridge, Massachusetts, fears that librarians may come under increasing pressure not to buy potentially controversial books. "If you are choosing a book for a library, you are a professional who understands children. If your judgment is not only questioned but really abused, next time books come around, there is the intimidation factor. The losers are the kids."
Most parents, she says, want sex education for their children because they share the concern that teenagers should be able to protect themselves against early pregnancy and disease. "You've got these 'Just Say No' campaigns and the kids are sitting in the shopping malls saying it's baloney."
Ms Harris recalls signing books in Orange county, an arch-conservative area of California where she expected barely anyone to turn up. "There was a huge turnout and one woman came up and said no one in her family or community knew she was there. She had an 11-year-old daughter and she was terrified what would happen to her because she went to a religious school, and the mother did not know how to talk to her about sex education.
"She said, 'I am going to invite all my daughter's friends around and read this book to them'. She ended up buying six copies."
What does Ms Harris think lies behind the new fervour of the moral majority? "There are some very deeply held religious beliefs that this is bad and immoral, and the Scriptures teach this. If your children experience this, they will become wayward, damned. You can stop your children becoming bad children if you don't let them get this information. Therefore you stop it in the school and the library."
Ms Harris was originally asked if she would write a book on HIV and Aids, but rapidly became convinced that she ought to write about sex. Even in an educated society, she says, myths such as "you can't get pregnant the first time" endure.
"One of the latest figures I have heard is that almost 30 per cent of those developing Aids are in their late teens and early 20s. If that's the case, these children were infected because they didn't get any information before and during puberty."
Homosexuality is a great taboo. Horrified television reporters asked why she had given it a chapter in the book but grateful parents have been buying it for their gay sons and daughters.
The book is now sold in 15 countries. Ms Harris, currently in Britain to promote her latest book, Happy Birth Day!, an account of a newborn baby's first 24 hours, believes Britain has a "less uptight" attitude to sex.
However, children find their own way of learning, and school librarians have told Ms Harris of pupils reading the book intently for a session, then hiding it "among the Greek myths" awaiting further study.
Let's Talk About Sex by Robie H Harris (Walker, Pounds 7.99). Happy Birth Day! by Robie H Harris (Walker, Pounds 9.99)