namespace WpfApp1.Management
{
    public class BookManagement : INotifyPropertyChanged
    { ...

<Window
    x:Class="WpfApp1.MainWindow"
    x:Name="mainWindow"
    ...
    xmlns:mangmt="clr-namespace:WpfApp1.Management"

<Window.Resources>
    <mangmt:BookManagement x:Key="bookManagement" />
</Window.Resources>

BookManagement bm= Application.Current.Resources["bookManagement"] as BookManagement;
bm.SelectedTab = "summary";