// Here is the text I want to add.
string text = "Non-numeric text.";

// Find the SharedStringTable element and append my text to it.
var sharedStringTable = document.WorkbookPart.GetPartsOfType<SharedStringTablePart>().First().SharedStringTable;
var item = sharedStringTable.AppendChild(new SharedStringItem(new Text(text)));

// Set the data type of the cell to SharedString.
cell.DataType = new EnumValue<CellValues>(CellValues.SharedString);

// Set the value of the cell to the index of the SharedStringItem.
cell.CellValue = new CellValue(item.ElementsBefore().Count().ToString());

string alpha = "ABCDEFGHIJKLMNOPQRSTUVQXYZ";
for (int colInx = 0; colInx < reader.FieldCount; colInx++)
{
    AppendTextCell(alpha[colInx] + "1", reader.GetName(colInx), headerRow);
}

private static void AppendTextCell(string cellReference, string cellStringValue, Row excelRow)
{
    //  Add a new Excel Cell to our Row
    Cell cell = new Cell() { CellReference = cellReference, DataType = new EnumValue<CellValues>(CellValues.String) };
    CellValue cellValue = new CellValue();
    cellValue.Text = cellStringValue.ToString();
    cell.Append(cellValue);
    excelRow.Append(cell);
}