I don't think that anyone on this thread thinks that legal texts on any area of law should be banned (and there are some really interesting books on law and sex in historical and international contexts out at the moment), but the purpose of this book is clearly to enable paedophile sex tourists to rape children.
Mostly we're familiar with liberal free speech values - all fine when they apply to 'self regarding activites' (which is what liberal free speech vlaues are based on). But raping children isn't really a self-regarding activity, is it?
I can see how your argument makes sense in that it protects speech that some people find offensive - OK, fine, well and good, why should anyone have a right not to be offended?
But that's different from speech that incites harmful behaviour - and traditionally this sort of speech isn't protected by (English) law or by liberal values. It seems quite clear that this book aims to enable rape of children. Therefore it's not just offensive, it's harmful. Not worth protecting legally or morally as 'free speech'.