"Dlaczego ona pisze książkę?"

Translation: Why is she writing a book?

[dropdbassett]

Dlaczego nie?

[va-diim]

It can either be,

"Why does she write bookS?" or,

"Why is she writing a book?".

[Jellei]

The second option you gave is correct, but why do you want to change singular to plural in the first option? The original sentence has one, singular, book.

[va-diim]

I did that on purpose because "why does she write a book" is unnatural speech. It works with plural normally: "Why does she write books?" It also works normally with negation: "Why doesn't she write a book?"

The correct structure with a singular book is "Why IS she WRITING a book?" This is because, "She writes a book," is unnatural. It should be, "She is writing a book."

[Jellei]

Okay, this is a very good point. You should probably report it.

[deactivated user]

I'd say "Why is she writing a book?" sounds better. "Why does she write a book?" sounds like an odd construction even though it's not ungrammatical.

[Jellei]

Changed the main sentence to "Why is she writing a book?" :)