Learn English – What’s the difference between “two story books” and “two story-books”

Sometimes when an adjective + noun combination is used very frequently and becomes very common, we make the transition from writing the two as separate words and make them one combined word.

So initially I'm willing to bet that "story book" was correct, just as "picture book" is two words, and "recipe book" is. It is a book that contains recipes, pictures, or stories. This makes perfect sense. But over time when the combination of words becomes very common, we can see evolution such as "story book" to "storybook". "Story book" is no longer correct; we always use "storybook". This hasn't happened to "recipe books" or "picture books", though, for which we always write the words separately. But check back in a couple hundred years and that might have changed. English is always evolving! There's no real way to know which words are written separately and which as a single word, except to learn and memorize them.

Regarding the hyphen, it would never be used in a case like this. We do not use a hyphen to connect the adjective to the noun in the simple case. A story-book doesn't make any sense. However we do use hyphens to connect compound adjectives which then refer to a noun. So if you are talking about a single book which contains two stories, you would call it a "two-story book". That is, a book which contains two stories. If you are referring to two items which are called storybooks, you would simply say "two storybooks". That is, these are storybooks, and I have two of them.