On the rating point - As far as I know there's no requirement to provide a recommended age rating for books, so I guess it's at least somewhat thoughtful of them to even bother. It sounds like 16+ is probably a fair advisory, given that sex scenes and nudity are allowed in 15 rated films. Hell, you can have sex when you're 16 (but not view pornography, even fictional cartoon pornography, until you're 18, make sense of that if you can). Where drawings become pornography is a very blurred line - Alan Moore and Melinda Gebbie's Lost Girls for example isn't considered pornography but it does feature explicit drawings which, if they were photographs or even animated, would be considered pornography.