HI,

try like this. it may help you



 
                      Text='<%# Limit(Eval("BookSynopsis"),100) %>'
                Tooltip='<%# Eval("BookSynopsis") %>'>
     
                      Text="Read More"
                Visible='<%# SetVisibility(Eval("BookSynopsis"), 100) %>'
                OnClick="ReadMoreLinkButton_Click">
     
 


And code-behind

protected bool SetVisibility(object Desc, int length)
{
    return Desc.ToString().Length > length;
}

protected void ReadMoreLinkButton_Click(object sender, EventArgs e)
{
    LinkButton button = (LinkButton)sender;
    GridViewRow row = button.NamingContainer as GridViewRow;
    Label descLabel = row.FindControl("Label1") as Label;
    button.Text = (button.Text == "Read More") ? "Hide" : "Read More";
    string temp = descLabel.Text;
    descLabel.Text = descLabel.ToolTip;
    descLabel.ToolTip = temp;
}

//including the original helper from the question with changes in signature

protected string Limit(object Desc, int length)
{
    StringBuilder strDesc = new StringBuilder();
    strDesc.Insert(0, Desc.ToString());

    if (strDesc.Length > length)
        return strDesc.ToString().Substring(0, length) + "..." + [Read More];
    else return strDesc.ToString();
}