<Window x:Class="WpfApp1.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:WpfApp1"
     mc:Ignorable="d"
     Title="MainWindow" Height="450" Width="800">
     <Grid>
          <Grid.ColumnDefinitions>
               <ColumnDefinition Width="auto"/>
               <ColumnDefinition Width="auto"/>
          </Grid.ColumnDefinitions>

          <!-- textbox with binding text-->
          <TextBox Text="{Binding BookName}" Grid.Column="0" Height="25" Width="250" />

          <!-- button-->
          <Button Content="Click me" Grid.Column="1" Height="25" Width="100" Click="MyButtonClick1" />
     </Grid>
</Window>

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 WpfApp1
{
     /// <summary>
     /// Interaction logic for MainWindow.xaml
     /// </summary>
     public partial class MainWindow : Window
     {
          public MainWindow()
          {
               InitializeComponent();

               // for binding
               this.DataContext = this;
          }

          private string _bookName = "Lords of the rings";
          public string BookName
          {
               get
               {
                    return _bookName;
               }
               set { _bookName = value; }
          }

          private void MyButtonClick1(object sender, RoutedEventArgs e)
          {
               MessageBox.Show(_bookName);
          }
     }
}