Tkinter object scope

[Gribouillis]

The solution is to make the widgets that you want to access later members of the EnterBookWindow instance, for example in __init__()

self.entBookTitle = ttk.Entry(...)

Later

    def SaveBook(self):
        book = Book(
            self.entBookTitle.get(), self.entBookISBN.get(), self.entBookAuthor.get(), self.entBookYear.get())

Don't use the global keyword. It generally means that you are not organizing the data correctly.