Get me a collection from XML.

using System.Collections.Generic;
using System.Windows;
using System.Xml;
using System.Xml.Linq;

namespace WpfApplication3
{
  public partial class MainWindow : Window
  {
    public MainWindow()
    {
      Library = new Library();
      InitializeComponent();
    }
    public Library Library { get; private set; }
  }

  public class Library
  {
    private List<Book> Stacks = new List<Book>();
    private Dictionary<string, Author> Authors = new Dictionary<string, Author>();
    public Library()
    {
      using (XmlReader reader = new XmlTextReader("../../Library.xml"))
      {
        foreach (XElement bookElement in XDocument.Load(reader).Element("Library").Elements())
        {
          Book aBook = new Book(bookElement.Element("Title").Value);
          Stacks.Add(aBook);
          foreach (var authorElement in bookElement.Elements("Author"))
          {
            string authorName = authorElement.Value;
            Author author;
            if (!Authors.TryGetValue(authorName, out author))
            {
              author = new Author(authorName);
              Authors.Add(authorName, author);
            }
            aBook.Authors.Add(author);
            author.Books.Add(new IndexedItem<Book>(author.Books.Count + 1, aBook));
          }
        }
      }
    }
    public IEnumerable<Author> AllAuthors
    { get { return Authors.Values; } }
  }

  public class Book
  {
    public Book(string title)
    {
      Title = title;
      Authors = new List<Author>();
    }
    public string Title { get; private set; }
    public List<Author> Authors { get; private set; }
  }

  public class Author
  {
    public Author(string name)
    {
      Name = name;
      Books = new List<IndexedItem<Book>>();
    }
    public List<IndexedItem<Book>> Books { get; private set; }
    public string Name { get; private set; }
  }

  public class IndexedItem<T>
  {
    public IndexedItem(int index, T item)
    {
      Index = index;
      Item = item;
    }
    public int Index { get; private set; }
    public T Item { get; private set; }
  }
}

<Window x:Class="WpfApplication3.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:local="clr-namespace:WpfApplication3"
        Title="MainWindow"
        Height="350"
        Width="525"
        DataContext="{Binding Path=Library, RelativeSource={x:Static RelativeSource.Self}}">
  <ListBox x:Name="topBox"
           ItemsSource="{Binding AllAuthors}">
    <ListBox.ItemTemplate>
      <DataTemplate>
        <StackPanel Orientation="Vertical">
          <TextBlock x:Name="Name"
                     Text="{Binding Path=Name}"
                     HorizontalAlignment="Left"
                     VerticalAlignment="Center"
                     Margin="0,0,10,0" />
          <ListBox ItemsSource="{Binding Books}">
            <ListBox.ItemTemplate>
              <DataTemplate>
                <StackPanel Orientation="Horizontal">
                  <TextBlock x:Name="BookNumber"
                             Text="{Binding Path=Index}"
                             HorizontalAlignment="Left"
                             VerticalAlignment="Center"
                             Margin="10,0" />
                  <TextBlock x:Name="Title"
                             Text="{Binding Path=Item.Title}"
                             HorizontalAlignment="Left"
                             VerticalAlignment="Center" />
                </StackPanel>
              </DataTemplate>
            </ListBox.ItemTemplate>
          </ListBox>
        </StackPanel>
      </DataTemplate>
    </ListBox.ItemTemplate>
  </ListBox>
</Window>