Problem z bindowaniem do ViewModel

View

<UserControl.DataContext>
        <ViewModels:ShelfViewModel/>
    </UserControl.DataContext>
    <Grid>
        <Grid.RowDefinitions>
            <RowDefinition Height="0.2*"/>
            <RowDefinition/>
        </Grid.RowDefinitions>

        <StackPanel Orientation="Horizontal">
            <StackPanel.Resources>
                <ResourceDictionary Source="Resources/Images.xaml"/>
            </StackPanel.Resources>
            <Button Content="{StaticResource Img_AddSquareSmall}" Command="{Binding AddBookCommand}"/>
        </StackPanel>
        <ListBox Grid.Row="1" ItemsSource="{Binding Books}">
            <ListBox.ItemTemplate>
                <DataTemplate>
                    <StackPanel>
                        <Image Source="{Binding Path=CoverImage}"/>
                        <TextBlock Text="{Binding Path=Title}"/>
                    </StackPanel>
                </DataTemplate>
            </ListBox.ItemTemplate>
        </ListBox>
    </Grid>
</UserControl>

ViewModel:

public class ShelfViewModel : ViewModelBase
    {
        readonly IShelfService ShelfService;
        readonly ObservableCollection<EpubBook> Books;

        public ICommand AddBookCommand;
        public ICommand RemoveBookCommand;

        public ShelfViewModel()
        {
            ShelfService = ServiceLocator.Instance.GetService<IShelfService>();
            Books = ShelfService.Books;

            InitializeCommands();
        }

        void InitializeCommands()
        {
            AddBookCommand = new RelayCommand(AddBook);
        }

        void AddBook(object p)
        {
            ShelfService.LoadBook();
        }
    }

Konstruktor w VM się wykonuje, ale AddBook(object) nigdy nie jest wykonywane.

Ale ICommand AddBookCommand powinno być chyba propertisem. Czyli mieć get oraz set. Generalnie wszystko co się binduje powinno mieć get oraz set. Tak mi się wydaje.