Assuming it is a book about English, it would be "Piszemy książkę o [języku] angielskim", but I feel most of the times a native speaker(me for sure) would be more specific here, so it would be, for example, "Piszemy książkę o gramatyce angielskiego"(We are writing a book about English grammar), "Piszemy książkę o historii angielskiego(history of English [language]), "[…]o fonologii angielskiego"(English phonology) and so on, depending on the topic.
Prepositions do not change, so they do not have cases.
Words after preposition take different cases depending on preposition and context.
And here in "po ....u" it's not a real case, it's a special word form of adjective just for this kind of expressions.
I did write somewhere a list of possible meanings of po+ adjective, but it's basically- like we they do it there .