WPF datagrid implement multi select via behavior selectionchanged event in MVVM

<DataGrid ItemsSource="{Binding BooksCollection,Mode=TwoWay,UpdateSourceTrigger=PropertyChanged}"
          CanUserAddRows="False"
          AutoGenerateColumns="False"
          SelectionMode="Extended">
    <behavior:Interaction.Triggers>
        <behavior:EventTrigger EventName="SelectionChanged">
            <behavior:CallMethodAction MethodName="DataGrid_SelectionChanged" 
                                       TargetObject="{Binding}"/>
        </behavior:EventTrigger>
    </behavior:Interaction.Triggers>


public void DataGrid_SelectionChanged(object sender, SelectionChangedEventArgs e)
{
    var dg = e.OriginalSource as DataGrid;
    if(dg!=null)
    {
        var items = dg.SelectedItems?.Cast<Book>().ToList();
        if(items!=null && items.Any())
        {
            MessageBox.Show($"Selected {items.Count()}!\n");
        }
    }
}

//xaml
<Window x:Class="WpfApp26.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:behavior="http://schemas.microsoft.com/xaml/behaviors"
        xmlns:local="clr-namespace:WpfApp26"
        mc:Ignorable="d"
        WindowState="Maximized"
        Title="MainWindow" Height="450" Width="800">
    <Window.DataContext>
        <local:BookVM/> 
    </Window.DataContext>
    <Window.Resources>
        <local:SizeConverter x:Key="sizeConverter"/>
    </Window.Resources>
    <Grid>
        <DataGrid ItemsSource="{Binding BooksCollection,Mode=TwoWay,UpdateSourceTrigger=PropertyChanged}"
                  CanUserAddRows="False"
                  AutoGenerateColumns="False"
                  SelectionMode="Extended">
            <behavior:Interaction.Triggers>
                <behavior:EventTrigger EventName="SelectionChanged">
                    <behavior:CallMethodAction MethodName="DataGrid_SelectionChanged" 
                                               TargetObject="{Binding}"/>
                </behavior:EventTrigger>
            </behavior:Interaction.Triggers>
            <DataGrid.Columns>
                <DataGridTextColumn Header="Id" Binding="{Binding Id}"/>
                <DataGridTextColumn Header="ISBN" Binding="{Binding ISBN}"/>
                <DataGridTemplateColumn Header="Image">
                    <DataGridTemplateColumn.CellTemplate>
                        <DataTemplate>
                            <Border 
                                Width="{Binding ActualWidth,RelativeSource={RelativeSource Mode=FindAncestor,AncestorType={x:Type Window}},
                                Converter={StaticResource sizeConverter}}"
                                Height="{Binding ActualHeight,RelativeSource={RelativeSource Mode=FindAncestor,AncestorType={x:Type Window}},
                                Converter={StaticResource sizeConverter}}">
                                <Border.Background>
                                    <ImageBrush ImageSource="{Binding ImgSource}" Stretch="Uniform" />
                                </Border.Background>
                            </Border>
                        </DataTemplate>
                    </DataGridTemplateColumn.CellTemplate>
                </DataGridTemplateColumn>
            </DataGrid.Columns>
        </DataGrid>
    </Grid>
</Window>




//cs
using System;
using System.Collections.Generic;
using System.Collections.ObjectModel;
using System.ComponentModel;
using System.Linq;
using System.Security.Policy;
using System.Text;
using System.Threading.Tasks;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Data;
using System.Windows.Documents;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Media.Imaging;
using System.Windows.Navigation;
using System.Windows.Shapes;
using System.IO;
using System.Data.SqlTypes;
using System.Globalization;

namespace WpfApp26
{
    /// <summary>
    /// Interaction logic for MainWindow.xaml
    /// </summary>
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
        }
    }

    public class BookVM: INotifyPropertyChanged
    {
        private List<string> imgsList { get; set; }
        private int imgsCount = 0;

        public BookVM()
        {
            InitData();
        }

        private void InitData()
        {
            var imgs = Directory.GetFiles(@"../../Images");
            if (imgs != null && imgs.Any())
            {
                imgsCount = imgs.Count();
                imgsList = new List<string>(imgs);
                BooksCollection = new ObservableCollection<Book>();
                for(int i=0;i<10000;i++)
                {
                    Book book = new Book()
                    {
                        Id=i+1,
                        ISBN=$"ISBN_{i+1}",
                        Name=$"Name_{i+1}",
                        Topic=$"Topic_{i+1}",
                        ImgUrl = $"{imgs[i%imgsCount]}",
                        ImgSource=GetImgSource(imgs[i%imgsCount])
                    };
                    BooksCollection.Add(book);
                }
            }
        }

        private ImageSource GetImgSource(string url)
        {
            BitmapImage bmi = new BitmapImage();
            bmi.BeginInit();
            
            bmi.UriSource=new Uri(url, UriKind.RelativeOrAbsolute);
            bmi.EndInit();
            if(bmi.CanFreeze)
            {
                bmi.Freeze();
            }
            return bmi;
        }

        public void DataGrid_SelectionChanged(object sender, SelectionChangedEventArgs e)
        {
            var dg = e.OriginalSource as DataGrid;
            if(dg!=null)
            {
                var items = dg.SelectedItems?.Cast<Book>().ToList();
                if(items!=null && items.Any())
                {
                    MessageBox.Show($"Selected {items.Count()}!\n");
                }
            }
        }

        public event PropertyChangedEventHandler PropertyChanged;
        private void OnPropertyChanged(string propertyName)
        {
            var handler = PropertyChanged;
            if(handler!=null)
            {
                handler?.Invoke(this, new PropertyChangedEventArgs(propertyName));
            }
        }

        private ObservableCollection<Book> booksCollection;
        public ObservableCollection<Book> BooksCollection
        {
            get
            {
                if (booksCollection==null)
                {
                    booksCollection = new ObservableCollection<Book>();
                }
                return booksCollection;
            }
            set
            {
                if(booksCollection!=value)
                {
                    booksCollection = value;
                    OnPropertyChanged(nameof(BooksCollection));
                }
            }
        }
    }

    public class SizeConverter : IValueConverter
    {
        public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
        {
            double d = 0;
            if(double.TryParse(value?.ToString(), out d))
            {
                return d / 2;
            }
            return d;
        }

        public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
        {
            throw new NotImplementedException();
        }
    }

    public class Book
    {
        public int Id { get; set; }

        public string Name { get; set; }

        public string Topic { get; set; }

        public string ImgUrl { get; set; }

        public string ISBN { get; set; }

        public ImageSource ImgSource { get; set; }
    }
}