public void DeleteBook_NonExistingBook_ReturnsNotFound()
        {
            mockService.Setup(s => s.DeleteBook(It.IsAny <int>()))
            .Returns(BookTestData.DeleteBookServiceResponseNullData());

            BookController bookController = new BookController(mockService.Object);

            var result = bookController.DeleteBook(1);

            Assert.That(result, Is.InstanceOf <NotFoundObjectResult>());
        }