The method throws an OutOfStockException when no books are available with that title and author or a InsufficientCreditException if the customer has nosufficientfunds to pay for the book.
当不存在与title和author匹配的书籍时，该方法抛出Out Of StockException，或者当客户没有足够资金为书籍付款的情况下，该方法抛出InsufficientCreditException。