It's not a cedilla, even in English it's called "ogonek" (little tail).
ą is a nasalized 'o', so the way the letter looks is actually pretty confusing, because it's not really similar to 'a' in sound.
ę is a nasalized 'e', so it's more logical ;) However, if it's the final sound of the word, it's pronounced like a basic 'e'. If it's in the middle of the word, then it should be clearly nasalized.
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.