In order to get the value of textbox which is embedded within a gridview, You need to find row index in which the textbox is present and you need to know the command arguement which fires the event and the rowcommand_event.
We derive the row index using the following code:
int rowIndex = ((GridViewRow((LinkButton)e.CommandSource).NamingContainer).
RowIndex;
After getting the row index we need to know the id of the textbox whose text we want to get let us assume that our textbox has an id "txtTitle" and we derive the text it contains as follows:
string title = ((TextBox)GridView1.Rows[rowIndex].FindControl("txtTitle")).Text;