void ReadExcelFileDOM(string fileName)
{
    var filePath = fileName;
    string result = "";
    using (var document = SpreadsheetDocument.Open(filePath, false))
    {
        var workbookPart = document.WorkbookPart;
        var workbook = workbookPart.Workbook;

        var sheets = workbook.Descendants<Sheet>();
        int i = 0;

        var sheet = sheets.Where(s => s.Name == "CORE GAME").ToList()[0];

        var worksheetPart = (WorksheetPart)workbookPart.GetPartById(sheet.Id);
        var sharedStringPart = workbookPart.SharedStringTablePart;
        var values = sharedStringPart.SharedStringTable.Elements<SharedStringItem>().ToArray();

        MessageBox.Show("values: " + values.Length);

        var cells = worksheetPart.Worksheet.Descendants<Cell>();

        foreach (var cell in cells)
        {
            try {
                // The cells contains a string input that is not a formula
                if (cell.DataType != null && cell.DataType.Value == CellValues.SharedString)
                {
                    var index = int.Parse(cell.CellValue.Text);
                    var value = values[index].InnerText;
                    richTextBox1.Text += value + " index "+index+"\n";
                    if(value.Contains("CHAPTER"))
                    {
                        richTextBox1.Text += "CHAPTER " + (i + 1);
                    }
                }
                else
                {
                    //result += cell.CellValue.Text + "\n";
                }
            }
            catch(Exception e) { }
        }
    }
}

Chapters
   Chapter 1
        Page 1
             Key = Value
             Key = Value
             Key = Value
             Key = Value
        Page 2
             Key = Value
             Key = Value
             Key = Value
             Key = Value
         ...
   Chapter 2
        Page 1
             Key = Value
             Key = Value
             Key = Value
             Key = Value