public partial class Catalog
{
    [System.Diagnostics.CodeAnalysis.SuppressMessage("Microsoft.Usage", "CA2214:DoNotCallOverridableMethodsInConstructors")]
    public Catalog()
    {
        this.Books = new HashSet<Book>();
    }

    public System.Guid Id { get; set; }
    public string Name { get; set; }

    [System.Diagnostics.CodeAnalysis.SuppressMessage("Microsoft.Usage", "CA2227:CollectionPropertiesShouldBeReadOnly")]
    public virtual ICollection<Book> Books { get; set; }
}


public partial class Book
{
    [System.Diagnostics.CodeAnalysis.SuppressMessage("Microsoft.Usage", "CA2214:DoNotCallOverridableMethodsInConstructors")]
    public Book()
    {
        this.Orders = new HashSet<Order>();
    }

    public System.Guid Id { get; set; }
    public string Name { get; set; }
    public string Genre { get; set; }
    public string Author { get; set; }
    public short Year { get; set; }

    public virtual Catalog Catalog { get; set; }
    [System.Diagnostics.CodeAnalysis.SuppressMessage("Microsoft.Usage", "CA2227:CollectionPropertiesShouldBeReadOnly")]
    public virtual ICollection<Order> Orders { get; set; }
}


public partial class Order
{
    [System.Diagnostics.CodeAnalysis.SuppressMessage("Microsoft.Usage", "CA2214:DoNotCallOverridableMethodsInConstructors")]
    public Order()
    {
        this.Books = new HashSet<Book>();
    }

    public System.Guid Id { get; set; }
    public string Number { get; set; }
    public Nullable<System.DateTime> RegisteredOn { get; set; }
    public Nullable<System.DateTime> ClosureDate { get; set; }

    public virtual Reader Reader { get; set; }
    [System.Diagnostics.CodeAnalysis.SuppressMessage("Microsoft.Usage", "CA2227:CollectionPropertiesShouldBeReadOnly")]
    public virtual ICollection<Book> Books { get; set; }
}


public partial class LibraryModelContainer : DbContext
{
    public LibraryModelContainer()
        : base("name=LibraryModelContainer")
    {
    }

    protected override void OnModelCreating(DbModelBuilder modelBuilder)
    {
        throw new UnintentionalCodeFirstException();
    }

    public virtual DbSet<Book> Books { get; set; }
    public virtual DbSet<Catalog> Catalogs { get; set; }
    public virtual DbSet<Order> Orders { get; set; }
    public virtual DbSet<Reader> Readers { get; set; }
}

using (LibraryModelContainer dbContainer = new LibraryModelContainer())
{
    Catalog catalog = new Catalog
    {
        Id = Guid.NewGuid(),
        Name = CatalogTextBox.Text
    };

    dbContainer.Catalogs.Add(catalog);
    dbContainer.SaveChanges();
}

using (LibraryModelContainer dbContainer = new LibraryModelContainer())
{
    Catalog catalog = dbContainer.Catalogs.Find(((Catalog)BookCatalogComboBox.SelectedItem).Id);
    Book book = new Book
    {
        Id = Guid.NewGuid(),
        Name = BookAuthorTextBox.Text,
        Genre = BookGenreComboBox.Text,
        Author = BookAuthorTextBox.Text,
        Year = Convert.ToInt16(BookYearMaskedTextBox.Text),
        Catalog = catalog
    };

    dbContainer.Books.Add(book);
    dbContainer.SaveChanges();
    _window.BookDataGrid.ItemsSource = dbContainer.Books.ToList();
}

using (LibraryModelContainer dbContainer = new LibraryModelContainer())
{
    Reader reader = dbContainer.Readers.Find(((Reader)ReaderNameComboBox.SelectedItem).Id);

    ICollection<Book> books = new List<Book>();

    foreach (Book book in BasketBookDataGrid.Items)
    {
        books.Add(book);
    }

    Order order = new Order
    {
        Id = Guid.NewGuid(),
        Number = "НОМЕР123",
        RegisteredOn = DateTime.Now,
        ClosureDate = null,
        Reader = reader,
        Books = books
    };

    dbContainer.Orders.Add(order);
    dbContainer.SaveChanges();
    _window.OrderDataGrid.ItemsSource = dbContainer.Orders.ToList();
}

<DataGrid Name="BookDataGrid">
    <DataGrid.Columns>
        <DataGridTextColumn Binding="{Binding Name}" Header="Название" MinWidth="114"></DataGridTextColumn>
        <DataGridTextColumn Binding="{Binding Genre}" Header="Жанр" MinWidth="114"></DataGridTextColumn>
        <DataGridTextColumn Binding="{Binding Author}" Header="Автор" MinWidth="114"></DataGridTextColumn>
        <DataGridTextColumn Binding="{Binding Year}" Header="Год издания" MinWidth="114"></DataGridTextColumn>
    </DataGrid.Columns>
</DataGrid>

private void AddBookToTheBasket_Click(object sender, RoutedEventArgs e) // Добавление книг в корзину заказа
{
    using (LibraryModelContainer dbContainer = new LibraryModelContainer())
    {
        Book book = dbContainer.Books.Find(((Book)BookDataGrid.SelectedItem).Id);
        if (book != null) // Если в DataGrid книг выбрана строка
        {
            BasketBookDataGrid.Items.Add(book);
        }
        else // Если в DataGrid книг ничего не выбрано
        {
            MessageBox.Show("В таблице книг не выбрана книга.");
        }
    }
}