private void btnAdd_Click(object sender, EventArgs e)
        {
            string bookName  = txtBookName.Text;
            string bookGenre = txtBookGenre.Text;
            string bookPrice = txtPrice.Text;
            //string bookAuthor = txtAuthorName.Text;
            string bookDescript = txtBookDascript.Text;

            // secilmish qrupun ID nomresinin  groupID-ye verilmesi
            string authorId = ((BookComboBox)cmbAuthor.SelectedItem).Value;

            // Author tipinden deyishen yaradiriq
            // ve author siyahisinin ID-lerinden secilmishini deyishene veririk
            Author selectedAuthor = AuthorList.GetAuthorByID(authorId);

            if (selectedAuthor == null) // eger secilmish author yoxdursa
            {
                MessageBox.Show("Author doesn't exist");
                return;
            }

            if (string.IsNullOrEmpty(bookName))
            {
                // ERROR mesajinin verilmersi
                MessageBox.Show("Book name is not valid", "Failed!",
                                MessageBoxButtons.OK, MessageBoxIcon.Error);

                return;
            }
            if (string.IsNullOrEmpty(bookGenre))
            {
                // ERROR mesajinin verilmersi
                MessageBox.Show("Book genre is not valid", "Failed!",
                                MessageBoxButtons.OK, MessageBoxIcon.Error);

                return;
            }
            if (string.IsNullOrEmpty(bookDescript))
            {
                // ERROR mesajinin verilmersi
                MessageBox.Show("Book description is not valid", "Failed!",
                                MessageBoxButtons.OK, MessageBoxIcon.Error);

                return;
            }
            if (string.IsNullOrEmpty(bookPrice))
            {
                // ERROR mesajinin verilmersi
                MessageBox.Show("Book price is not valid", "Failed!",
                                MessageBoxButtons.OK, MessageBoxIcon.Error);

                return;
            }

            // eger AuthorListde bu ad varsa
            if (BookList.ContainsBookName(bookName))
            {
                // ERROR mesaji verilir
                MessageBox.Show("Book name is duplicate", "Failed!",
                                MessageBoxButtons.OK, MessageBoxIcon.Error);

                return;
            }



            BookList.AddBook(new Book {
                BookName    = bookName,
                BookJenre   = bookGenre,
                Price       = bookPrice,
                Description = bookDescript,
                Authors     = selectedAuthor,
            });

            txtBookName.Text     = "";
            txtBookGenre.Text    = "";
            txtBookDascript.Text = "";
            txtPrice.Text        = "";
        }