And out fell a book.
What does out fell mean? Can it just be fell out?
"And out fell a book".
This is a stylistic inversion of: And a book fell out.
It emphasizes the idea of out. That is, a preposition involving a direction.
Another example: And in came a big dog. Inversion of: And a big dog came in.
These emphatic inversions often occur with action verbs involving movement of some kind.
Sometimes, they are somewhat abstract:
And out came the truth. And the truth came out.
They also occur with other prepositions: And in strode the guilty party. The guilty party strode in.
Common preposition are in, out, along, up, down, around. There are surely others. I can't provide an exhaustive list.
And down came the flag. And the flag came down.
The book fell. [Means the book fell from some place.
The book fell out. [Means the book fell out of a place, like a box, for example.]
These forms are most used in writing and storytelling.
Either "out fell a book" or "a book fell out" is equally grammatical and both versions are quite common. They mean the same thing, but have a slightly different register and slightly different emphasis.
Such inversion of subject and verb used to be a general possibility in older forms of English, since it used to be a V2-word-order language; however, it is now much more limited.
One case where inversion is still possible concerns what is often called "locative inversion." This refers to using an expression related to location in the "first position," followed by the verb, and then followed by a noun that is being introduced into the conversation. These are a type of presentational sentence that exist in most or all languages.
In the case of the two sentences at issue, the inversion allows the sentence stress to easily fall on the last and most important element. It forms a presentational sentence that focuses on the event described as the appearance of the book and tags the book as an entity new, but significant entity for the upcoming conversation. It is also a frequent choice after a conjunction (such as "and") introducing a change of focus.
The form without the inversion better matches the default order of modern English sentences (apart from inversions with "there is/are," which are still the usual choice for presentational sentences, as opposed to the subject-verb order), but requires a sentence stress on the first word. This is probably the most frequently found order, but the inversion sounds slightly more elegant in style. It is also a better choice if the words are pronounced slowly to heighten the suspense of what was going to fall out.