Excellibrary documentation

This question and its answers are really old. Anyone looking at this now - forget ExcelLibrary. NPOI is now the way to go, and works well for both .xls and .xlsx

https://github.com/nissl-lab/npoi - where to get the C# downloads
https://poi.apache.org/ - the best documentation I've found, even though it is the Java version.

Try this:

var workbook = Workbook.Load("spreadsheet.xls");
var worksheet = workbook.Worksheets[0]; // assuming only 1 worksheet
var cells = worksheet.Cells;
var dataTable = new DataTable("datatable");

// add columns
dataTable.Columns.Add("column1");
dataTable.Columns.Add("column2");
...

// add rows
for (int rowIndex = cells.FirstRowIndex + 1; rowIndex <= cells.LastRowIndex; rowIndex++)
{
    var values = new List<string>();
    foreach(var cell in cells.GetRow(rowIndex))
    {
        values.Add(cell.Value.StringValue);
    }

    dataTable.LoadDataRow(values.ToArray(), true);
}

It's not exactly the prettiest code but it returns a DataTable. I recommend that you just use the values directly if possible ie. instead of converting to a DataTable read the values directly and skip this conversion step.

//create new xls file
string file = "C:\newdoc.xls";
Workbook workbook = new Workbook(); 
Worksheet worksheet = new Worksheet("First Sheet"); 
worksheet.Cells[0, 1] = new Cell((short)1); 
worksheet.Cells[2, 0] = new Cell(9999999); 
worksheet.Cells[3, 3] = new Cell((decimal)3.45); 
worksheet.Cells[2, 2] = new Cell("Text string"); 
worksheet.Cells[2, 4] = new Cell("Second string"); 
worksheet.Cells[4, 0] = new Cell(32764.5, "#,##0.00"); 
worksheet.Cells[5, 1] = new Cell(DateTime.Now, @"YYYY-MM-DD"); worksheet.Cells.ColumnWidth[0, 1] = 3000; 
workbook.Worksheets.Add(worksheet); workbook.Save(file);

// open xls file
Workbook book = Workbook.Load(file);
Worksheet sheet = book.Worksheets[0];

// traverse cells
foreach (Pair, Cell> cell in sheet.Cells)
{ 
    dgvCells[cell.Left.Right, cell.Left.Left].Value = cell.Right.Value;
}

// traverse rows by Index
for (int rowIndex = sheet.Cells.FirstRowIndex; rowIndex <= sheet.Cells.LastRowIndex; rowIndex++)
{
    Row row = sheet.Cells.GetRow(rowIndex); 
    for (int colIndex = row.FirstColIndex; colIndex <= row.LastColIndex; colIndex++)
    {
        Cell cell = row.GetCell(colIndex);
    }
}