<ComboBox Grid.Row="0" Grid.Column="0" Grid.ColumnSpan="1" Name="firstCombo"
            DisplayMemberPath="AuthorName" ItemsSource="{Binding Authors}" SelectedItem="{Binding SelectedAuthor}" />
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        public Author SelectedAuthor
        {
            get
            {
                return this.selectedAuthor;
            }
            set
            {
                this.selectedAuthor = value;
                this.FillBooks();
            }
        }