WPF ComboBox multiselect via ControlTemplate of ComboxItem

<Window x:Class="WpfApp99.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:local="clr-namespace:WpfApp99"
        WindowState="Maximized"
        mc:Ignorable="d"
        Title="MainWindow" Height="450" Width="800">
    <Window.Resources>
        <Style TargetType="{x:Type TextBlock}" x:Key="tbkStyle">
            <Setter Property="TextWrapping" Value="Wrap"/>            
        </Style>
        <Style TargetType="ComboBoxItem" x:Key="cbxItemStyle">
            <Setter Property="Template">
                <Setter.Value>
                    <ControlTemplate TargetType="{x:Type ComboBoxItem}">
                        <Border Height="100" Width="1300">
                            <Grid>
                                <Grid.ColumnDefinitions>
                                    <ColumnDefinition Width="100"/>
                                    <ColumnDefinition Width="100"/>
                                    <ColumnDefinition/>
                                </Grid.ColumnDefinitions>
                                <CheckBox Grid.Column="0" IsChecked="{Binding IsSTEM}" 
                                          IsThreeState="False"/>
                                <TextBlock Grid.Column="1" Text="{Binding Id}" />
                                <TextBlock Grid.Column="2" Text="{Binding ISBN}" 
                                           Style="{StaticResource tbkStyle}"/>
                            </Grid>
                            <Border.Background>
                                <ImageBrush ImageSource="{Binding ImgSource}"
                                            RenderOptions.BitmapScalingMode="Fant"
                                            Stretch="Uniform"/>
                            </Border.Background>
                        </Border>
                        <ControlTemplate.Triggers>
                            <Trigger Property="IsMouseOver" Value="True">
                                <Setter Property="Foreground" Value="Red"/>
                                <Setter Property="FontSize" Value="30"/>
                            </Trigger>
                        </ControlTemplate.Triggers>
                    </ControlTemplate>                    
                </Setter.Value>
            </Setter>
        </Style>
    </Window.Resources>
    <Grid>
        <ComboBox Width="1300" 
                  Height="100"
                  MaxDropDownHeight="700"
                  HorizontalAlignment="Left"
                  VerticalAlignment="Top"
                  ItemsSource="{Binding BooksCollection,Mode=TwoWay,UpdateSourceTrigger=PropertyChanged}"
                  ItemContainerStyle="{StaticResource cbxItemStyle}"/>
    </Grid>
</Window>



//cs
using System.Collections.ObjectModel;
using System.ComponentModel;
using System.IO;
using System.Runtime.CompilerServices;
using System.Text;
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;

namespace WpfApp99
{
    /// <summary>
    /// Interaction logic for MainWindow.xaml
    /// </summary>
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
            var vm = new BookVM();
            this.DataContext = vm;
        }
    }

    public class BookVM : INotifyPropertyChanged
    {
        public BookVM()
        {
            InitData();
        }

        private void InitData()
        {
            var imgs = Directory.GetFiles(@"../../../Images");
            if (imgs != null && imgs.Any())
            {
                int imgsCount = imgs.Count();
                BooksCollection = new ObservableCollection<Book>();
                for (int i = 0; i < imgsCount; i++)
                {
                    BooksCollection.Add(new Book()
                    {
                        Id = i + 1,
                        IsSelected=false,
                        ISBN = $"ISBN_{Guid.NewGuid().ToString("N")}",
                        ImgSource = GetImageSourceViaUrl(imgs[i%imgsCount])
                    });
                }
            }
        }

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

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

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

    public class Book
    {
        public int Id { get; set; }
        public bool IsSelected { get; set; }
        public string ISBN { get; set; }
        public ImageSource ImgSource { get; set; }

    }
}