"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.
I believe this is wrong. The correct translation would be "you are all writing a book." The Polish phrase should be "piszesz książkę" for that translation to work.
Well, for most English users there is no difference between singular and plural 'you'. Only some dialects use "you all" or "you guys".
Well, to be honest, I can't think of anywhere where "you" would be used as plural. There is the colloquialism "youse" and the two constructions you offer. Nowhere is "you" used as a plural. Nowhere in the English speaking world would "you are writing a book" be used in reference to a group.
I beg to differ. Creative writing classes, author's conferences, maybe doctors or scientists conferences where there are multiple people writing a book/books and are being addressed. It IS used. "No where is 'you' used as plural" that is so incorrect i dont know where to begin with that. Its like you didnt spend at least 12 years in school with the teacher adressing the class as "you".