SpreadsheetGear for .NET reads and writes CSV / XLS / XLSX and does more.
You can see live ASP.NET samples with C# and VB source code here and download a free trial here.
Of course I think SpreadsheetGear is the best library to import / export Excel workbooks in ASP.NET - but I am biased. You can see what some of our customers say on the right hand side of this page.
Disclaimer: I own SpreadsheetGear LLC