<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">
     <StackPanel Orientation="Vertical" Margin="40 20 0 0">

          <!-- enter text -->
          <TextBox Name="ElementWithText" HorizontalAlignment="Left" Width="300" Height="20">
               <TextBox.Text>
                    <Binding Path="BookName"
                    UpdateSourceTrigger="PropertyChanged"
                    ValidatesOnDataErrors="True"
                    NotifyOnValidationError="True">
                         <Binding.ValidationRules>
                              <local:MyFieldValidation />
                         </Binding.ValidationRules>
                    </Binding>
               </TextBox.Text>
          </TextBox>

          <!-- Validation Error -->
          <Label Margin="0" Foreground="Red" Content="{Binding Path=(Validation.Errors)[0].ErrorContent, ElementName=ElementWithText}">
          </Label>

     </StackPanel>
</Window>

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
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
{
     public partial class MainWindow : Window
     {
          public string BookName { get; set; }

          public MainWindow()
          {
               InitializeComponent();

               // for binding
               this.DataContext = this;
          }
     }

     public class MyFieldValidation : ValidationRule
     {
          public override ValidationResult Validate(object value, System.Globalization.CultureInfo cultureInfo)
          {
               string text = (string)value;

               if (!string.IsNullOrEmpty(text))
               {
                    for (int i = 0; i < text.Length; i++)
                    {
                         if (!Char.IsLetter(text[i]))
                         {
                              // show error message
                              return new ValidationResult(false, "Error!, please enter correct book name, should be only letters");
                         }
                    }
               }
               return new ValidationResult(true, null);
          }
     }
}

<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">
     <StackPanel Orientation="Vertical" Margin="40 20 0 0">

          <!-- enter text -->
          <TextBox Name="ElementWithText" HorizontalAlignment="Left" Width="300" Height="20">
               <TextBox.Text>
                    <Binding Path="BookName"
                    UpdateSourceTrigger="PropertyChanged"
                    ValidatesOnDataErrors="True"
                    NotifyOnValidationError="True">
                         <Binding.ValidationRules>
                              <local:MyFieldValidation />
                         </Binding.ValidationRules>
                    </Binding>
               </TextBox.Text>
          </TextBox>

          <!-- Validation Error -->
          <Label Margin="0" Foreground="Red" Content="{Binding Path=(Validation.Errors)[0].ErrorContent, ElementName=ElementWithText}">
               <Label.Style>
                    <Style TargetType="Label">
                         <Style.Triggers>
                              <DataTrigger Binding="{Binding Path=(Validation.Errors)[0].ErrorContent, ElementName=ElementWithText}" Value="{x:Null}">
                                   <Setter Property="Visibility" Value="Collapsed" />
                              </DataTrigger>
                         </Style.Triggers>
                    </Style>
               </Label.Style>
          </Label>
    
          <!-- enter text -->
          <TextBox HorizontalAlignment="Left" Width="300" Height="20">
          </TextBox>

     </StackPanel>
</Window>