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);
}