My understanding is that there's no law against *buying* a book from an overseas retailer online, any more than there is a law against bringing it back in your suitcase. Provided it's under $1000 at a time. What there is a law against is parallel importing, which I believe only applies to commercial quantities - so you can't buy things from amazon and on-sell them, but any law stopping you from purchasing a book online would also have to stop you bringing one in, in person, in hard copy.

There *are* laws against bringing in restricted items, in person or by mail. This includes erasers, electric flyswats and pornography, but not books unless they contain 'objectionable material', whatever THAT means.

... yeah, I read the customs website in considerable detail. I had my reasons.