Dynamically Add and Delete row in GridView 

.aspx code
-----------
<div>
 <asp:GridView ID="gvBooks" runat="server" ShowFooter="True"
                                AutoGenerateColumns="False" HeaderStyle-CssClass="header-column" onrowcommand="gvBooks_RowCommand"
                                onrowdatabound="gvBooks_RowDataBound" BorderStyle="None" GridLines="None">
                                <Columns>
                                    <asp:TemplateField HeaderText="Book Name">
                                        <ItemTemplate>
                                            <asp:TextBox ID="txtBookName" runat="server">
                                            </asp:TextBox>
                                        </ItemTemplate>
                                        <FooterTemplate>
                                            <asp:LinkButton ID="lbNewBook" runat="server" CausesValidation="false" CommandName="NewBook">New</asp:LinkButton>
                                        </FooterTemplate>
                                    </asp:TemplateField>
                                    <asp:TemplateField HeaderText="Author">
                                        <ItemTemplate>
                                            <asp:TextBox ID="txtAuthor" runat="server" Text='<%#Bind("Author") %>'></asp:TextBox>
                                        </ItemTemplate>
                                    </asp:TemplateField>
                                    <asp:TemplateField HeaderText="Description">
                                        <ItemTemplate>
                                            <asp:TextBox ID="txtDescription" runat="server" Text='<%#Bind("Description") %>'></asp:TextBox>
                                        </ItemTemplate>
                                    </asp:TemplateField>
                                    <asp:TemplateField HeaderText="Genre">
                                        <ItemTemplate>
                                            <asp:DropDownList ID="ddlGenre" runat="server" AppendDataBoundItems="true">
                                            <asp:ListItem>Choose...</asp:ListItem>
                                            <asp:ListItem>Computer</asp:ListItem>
                                            <asp:ListItem>Fantasy</asp:ListItem>
                                           <asp:ListItem>Horror</asp:ListItem>
                                           <asp:ListItem>Comedy</asp:ListItem>
                                            </asp:DropDownList>
                                        </ItemTemplate>
                                    </asp:TemplateField>
                                    <asp:TemplateField HeaderText="Price">
                                        <ItemTemplate>
                                            <asp:TextBox ID="txtPercentage" runat="server" Width="100px" Text='<%#Bind("Price") %>'></asp:TextBox>
                                        </ItemTemplate>
                                    </asp:TemplateField>
                                    <asp:TemplateField HeaderText="Publish Date">
                                        <ItemTemplate>
                                            <asp:TextBox ID="txtPublishDate" runat="server" Width="100px" Text='<%#Bind("PublishDate") %>'></asp:TextBox>
                                        </ItemTemplate>
                                    </asp:TemplateField>
                                    <asp:TemplateField>
                                        <ItemTemplate>
                                            <asp:ImageButton ID="imgbtnDeleteBook" runat="server" CausesValidation="false" ImageUrl="~/App_Themes/Red/Images/cross.png" CommandName="DeleteBook" />
                                        </ItemTemplate>
                                    </asp:TemplateField>
                                </Columns>
                            </asp:GridView>
                        </div>
.cs code

public partial class Default6 : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {
        if (!Page.IsPostBack)
        {
            AddBookDefault();
        }
    }

    private void AddBookDefault()
    {
        DataTable dt = new DataTable();
        DataRow dr = null;
        dt.Columns.Add(new DataColumn("BookName", typeof(string)));
        dt.Columns.Add(new DataColumn("Author", typeof(string)));
        dt.Columns.Add(new DataColumn("Description", typeof(string)));
        dt.Columns.Add(new DataColumn("Genre", typeof(string)));
        dt.Columns.Add(new DataColumn("Price", typeof(string)));
        dt.Columns.Add(new DataColumn("PublishDate", typeof(string)));

        dr = dt.NewRow();

        dr["BookName"] = string.Empty;
        dr["Author"] = string.Empty;
        dr["Description"] = string.Empty;
        dr["Genre"] = string.Empty;
        dr["Price"] = string.Empty;
        dr["PublishDate"] = string.Empty;

        dt.Rows.Add(dr);
        //dr = dt.NewRow();
        //Store the DataTable in ViewState

        ViewState["CurrentTable"] = dt;

        gvBooks.DataSource = dt;
        gvBooks.DataBind();
    }
    protected void gvBooks_RowCommand(object sender, GridViewCommandEventArgs e)
    {
        int rowIndex = 0;
        DataTable dt = (DataTable)ViewState["CurrentTable"];

        if (e.CommandName == "NewBook")
        {
            DataRow rw = dt.NewRow();
            dt.Rows.Add(rw);

            //Set Previous Data
            if (dt.Rows.Count > 0)
            {
                for (int i = 0; i < dt.Rows.Count - 1; i++)
                {
                    TextBox txtBookName = (TextBox)gvBooks.Rows[rowIndex].Cells[0].FindControl("txtBookName");
                    TextBox txtAuthor = (TextBox)gvBooks.Rows[rowIndex].Cells[1].FindControl("txtAuthor");
                    TextBox txtDescription = (TextBox)gvBooks.Rows[rowIndex].Cells[2].FindControl("txtDescription");
                    DropDownList ddlGenre = (DropDownList)gvBooks.Rows[rowIndex].Cells[3].FindControl("ddlGenre");
                    TextBox txtPrice = (TextBox)gvBooks.Rows[rowIndex].Cells[4].FindControl("txtPercentage");
                    TextBox txtPublishDate = (TextBox)gvBooks.Rows[rowIndex].Cells[5].FindControl("txtPublishDate");

                    dt.Rows[i]["BookName"] = txtBookName.Text;
                    dt.Rows[i]["Author"] = txtAuthor.Text;
                    dt.Rows[i]["Description"] = txtDescription.Text;
                    dt.Rows[i]["Genre"] = ddlGenre.SelectedItem.Text;
                    dt.Rows[i]["Price"] = txtPrice.Text;
                    dt.Rows[i]["PublishDate"] = txtPublishDate.Text;

                    rowIndex++;
                }
            }
        }
        else if (e.CommandName == "DeleteBook")
        {
            GridViewRow gvr = (GridViewRow)((ImageButton)e.CommandSource).NamingContainer;
            dt.Rows.RemoveAt(gvr.RowIndex);
        }

        gvBooks.DataSource = dt;
        gvBooks.DataBind();
    }

    protected void gvBooks_RowDataBound(object sender, GridViewRowEventArgs e)
    {
        if (e.Row.RowType == DataControlRowType.DataRow)
        {
            DropDownList ddlGenre = (DropDownList)e.Row.Cells[3].FindControl("ddlGenre");

            ddlGenre.SelectedValue = DataBinder.Eval(e.Row.DataItem, "Genre").ToString();
        }
    }

}