Hi elfoq,

Check the below insert code. Same you can change the update and delete code as per you page design.

SQL

CREATE TABLE BOOK
(
    ISBN INT PRIMARY KEY
)
CREATE TABLE Reader 
(
    ID_book INT PRIMARY KEY
)
CREATE TABLE Bridge 
(
    Id INT IDENTITY,
    ISBN INT FOREIGN KEY REFERENCES BOOK(ISBN),
    ID_book INT FOREIGN KEY REFERENCES Reader(ID_book)
)

HTML

<asp:TextBox runat="server" ID="txtBook" />
<asp:TextBox runat="server" ID="txtIDBook" />
<asp:Button Text="Save" runat="server" ID="btnSave" OnClick="OnSave" />

Namespaces

C#

using System.Configuration;
using System.Data.SqlClient;

VB.Net

Imports System.Configuration
Imports System.Data.SqlClient

Code

C#

protected void OnSave(object sender, EventArgs e)
{
    SqlCommand cmd = new SqlCommand();
    cmd.CommandText = "Insert Into BOOK VALUES(@ISBN)";
    cmd.Parameters.AddWithValue("@ISBN", txtBook.Text.Trim());
    // Insert In Book Table.
    Insert(cmd);

    cmd = new SqlCommand();
    cmd.CommandText = "Insert Into Reader VALUES(@ID_book)";
    cmd.Parameters.AddWithValue("@ID_book", txtIDBook.Text.Trim());
    // Insert In Reader Table.
    Insert(cmd);

    cmd = new SqlCommand();
    cmd.CommandText = "Insert Into Bridge VALUES(@ISBN,@ID_book)";
    cmd.Parameters.AddWithValue("@ISBN", txtBook.Text.Trim());
    cmd.Parameters.AddWithValue("@ID_book", txtIDBook.Text.Trim());
    // Insert In Bridge Table.
    Insert(cmd);
}

private void Insert(SqlCommand cmd)
{
    string conString = ConfigurationManager.ConnectionStrings["constr"].ConnectionString;
    using (SqlConnection con = new SqlConnection(conString))
    {
        cmd.Connection = con;
        con.Open();
        cmd.ExecuteNonQuery();
        con.Close();
    }
}

VB.Net

Protected Sub OnSave(ByVal sender As Object, ByVal e As EventArgs)
    Dim cmd As SqlCommand = New SqlCommand()
    cmd.CommandText = "Insert Into BOOK VALUES(@ISBN)"
    cmd.Parameters.AddWithValue("@ISBN", txtBook.Text.Trim())
    Insert(cmd)
    cmd = New SqlCommand()
    cmd.CommandText = "Insert Into Reader VALUES(@ID_book)"
    cmd.Parameters.AddWithValue("@ID_book", txtIDBook.Text.Trim())
    Insert(cmd)
    cmd = New SqlCommand()
    cmd.CommandText = "Insert Into Bridge VALUES(@ISBN,@ID_book)"
    cmd.Parameters.AddWithValue("@ISBN", txtBook.Text.Trim())
    cmd.Parameters.AddWithValue("@ID_book", txtIDBook.Text.Trim())
    Insert(cmd)
End Sub

Private Sub Insert(ByVal cmd As SqlCommand)
    Dim conString As String = ConfigurationManager.ConnectionStrings("constr").ConnectionString
    Using con As SqlConnection = New SqlConnection(conString)
        cmd.Connection = con
        con.Open()
        cmd.ExecuteNonQuery()
        con.Close()
    End Using
End Sub

For more details on CRUD refer below article.

Step by Step Tutorial for performing Insert Edit Update and Delete in GridView in ASP.Net