A style guide, basically. And as with any reasonably well constructed style guide it contain equal parts sage regard for the essential mutability of language and random prescriptivism, particularly on the subject of pronunciation.
The most famous and ancient of all misuses and not for that reason any less of a case of ignorant bullshit.
You always know where you stand, at least, though if you weren't a middle-aged white man from the South East of England that wasn't necessarily an enviable spot—entries on Womanese and the like have not, if we are being charitable, aged well. Though that in turn is a charity I suspect Amis would have rejected, as he clearly delighted in pushing buttons. If you're going to argue then you might as well argue about the important stuff.