XP Practices:
-Collective ownership
-coding standards
-sustainable pace
-test first
-acceptance tests
-automation
-onsite customer
-continuous integration
-pair programming

http://www.cyberarmy.net/library/article/202

product owner responsibility: Put into a simple list, typical product owner responsibilities are:

decide on the project vision - who is going to benefit from the project and how;
secure funding;...

so what a about the price

setVat(discount); setDiscount(vat);

package com.bookPortal.model;

public class Book {
private double basePrice = 0;
private int vat = 0;
private int discount = 0;
private double sellPrice;

public Book() {...

public class book {
private double basePrice = 0;
private double vat = 0;
private double discount = 0;
private double sellPrice;

public book() {
setBasePrice(0);
setVat(0);...

unittest problem