Píšem priamo sem, tak tam môžu byť chyby. Vychádzal som z príkladov na MSDN. Je to klasický ADO.NET kód:

var connectionString = @"Provider=Microsoft.Jet.OLEDB.4.0;Data Source=c:\data.xls;Extended Properties=Excel 8.0;";
using (var connection = new OleDbConnection(connectionString))
{
  connection.Open();
  using (var command = connection.CreateCommand())
  {
    command.CommandText = "select * from [Sheet1$]";
    using (var reader = command.ExecuteReader())
      while (reader.Read())
      {
        var value = reader.GetValue(0);
        // atď.
      }
  }
}