public void TestAverageRatingByAuthor()
        {
            Assert.AreEqual(4.23, books.AverageRatingByAuthor(b4), .003);
            //authors with multiple ratings
            Assert.AreEqual(3.79, books.AverageRatingByAuthor(b1), .003);
            //added another book by Charles Dickens to see if it adds correctly
            Book b11 = new Book("A CHristmas Carol", "Charles Dickens", 1843, 4.00);

            books.AddBook(b11);
            Assert.AreEqual(3.86, books.AverageRatingByAuthor(b1), .003);
        }