"You write a book" is accepted, but I think that with one book, focusing on the moment and saying "You are writing a book" is a lot more probable and logical.
In English, the word "You" can be used as a word to describe "Somebody" in a sentence such as "When you write a book..." =meaning= "When somebody writes a book...." Would there be a similar meaning in Polish using the plural "Wy"?
No, that would rather have to be some impersonal construction, more like "When one writes a book" (yeah, I understand that those can be possibly synonymous in English). So probably "Kiedy pisze się książkę..."
Well, although... okay, I guess in some contexts you could use it similarly in Polish... but rather with singular "ty" than plural "wy". I imagine it more as a guide, that addresses the reader directly. And plural... okay, what you see is clearly a stream of consciousness :D Plural would perhaps be possible if you were in class/lecture about writing books? So, what the heck, I guess it's possible.