Hi, well I see you have successfully retrieved the desired XmlNode. All that is left for you is to retrieve the required attribute from that node.
So something like this:
Dim bookAmoount = Book.Attributes("amoount").Value
Also just in case you're interested you can simplify your code with something like this:
Dim bookAmoount = doc.SelectSingleNode("//quote/quote_entry[@cet_id='A']").Attributes("amoount").Value