MS Excel files (2007) and above can be managed via OpenXML, whereas older formats (2003) were based on ExcelML, and need external MS add-in to convert to OpenXML format. This is a great tool to manage Microsoft Excel documents via OpenXML:
Another good tool is:
But if you want more flexiblity and ease-of-use while editing MS Excel files (97-2012 format) in .NET enviroment, then Aspose.Cells for .NET is your best bet. It is a great tool which can handle older as well as newer MS Excel format and even convert Excel files to PDF with 2-3 lines of code! The good thing about Aspose is that their tools have been tried and tested by a lot of customers and have the capability to be introduced in an enterprise environment.