This is the 197th most frequent German word.
dazu
"Dazu" means "to it," "for it," "in addition," or "with it," depending on context.
Referring to the relationship to 'a book'.
Ich habe ein Buch gelesen und nun möchte ich etwas dazu schreiben.
I read a book, and now I want to write something about it.
Uniting ideas, specifying inclusion.
Ich habe schon alles vorbereitet, dazu gehört auch die Dekoration.
I already prepared everything; that includes the decoration.
Handling a difficult topic or situation.