Possessive form or not? [duplicate]

Although Children's books is what everybody says, I would like to understand why the genitive case is applied in such case.

If I write books for children, children is an adjective here; not the owners of my book! The word "children" just defines or characterizes the type of books I write. Therefore, it's an adjective.

So, I understand that genitive/possessive case ("I write children's book") is incorrect grammar.

My question is: is the genitive case here really accepted as right? If I use "I write children books" (following the grammar principle) as as I say "I write pets books" (books about pets, and not possessions of pets) - would I be incorrect? Why?

There are some interesting points here. One is whether the apostrophe (’) serves any purpose, but we can leave that for another day.

We speak of children’s books, not because, in this context, the books belong to the children, but because the books are for children. Children’s acts as a modifier rather than a determiner. As the ‘Longman Grammar of Spoken and Written English’ says,

[such] genitives have the role of classifying the reference of the head noun: the question answered here is ‘What kind of X? . . . In many cases, a classifying genitive is equivalent to an adjective or a noun modifier . . .

To take up that last point, children’s books could, just about, be replaced with juvenile books or junior books. We can certainly speak of adult literature as well as adults' literature.

I don’t think pets books would normally be found. Instead you might possibly see a section in a bookshop for pet books. A better example might be animal books (not animals' books), which clearly refers to books about animals rather than books for animals.

Grammar is as it is, not as you would like it to be.

A story book, a picture book, a fiction book are all grammatical, but a children book is not, at least in the sense you mean it (It is possibly grammatical in the sense of a book about children, but is not in common use even in that sense).

The problem here is that basically "children's books" is an idiom. It is not incorrect to put two nouns together, but sometimes it goes against an established convention.

For instance "white and black photograph" is not incorrect, but it is strange because we only ever hear "black and white photograph".

A "driver's license" could be a "driver license" or "driving license". But it isn't, and that's that. It sounds strange because we are accustomed to using "driver's license" as a canned phrase.

"Children's book" has a more specific meaning, that the book belongs to children, and isn't about children.

Compare with "people's republic", which says something different from "people republic".

If we use A B, where A and B are nouns, there is a relationship between A and B in that an A B is a kind of B, which is somehow restricted to a narrower set, as qualified by A.

But A's B is more specific. There is a relationship bewteen A and B such that A's B is a kind of B, which belongs to A.

A "children book" or "child book" could be about children (for example rearing them) rather than for children, the way "car manual" is about a car (whereas the "owner's manual" is clearly not about the owner).