What’s the most “catch-all” way to manipulate Excel files?

I’ve been looking up excel libraries to deal with reading and writing excel files, and besides the Interop namespace, the most popular library for excel files seems to be EPPlus. I ended up using EPPlus for another project and found it relatively easy to use, so I wanted to incorporate it in a revamp of a previous project. However, I’ve learned that EPPlus does not interact with .xls files and my workplace still gets a volume of .xls files alongside .xlsx/.xlsm files.

One idea I was playing with was incorporating the Interop namespace AND EPPlus and choosing one based on the file extension, extract what I need, and then ride EPPlus all the way to the spreadsheet output. But I didn’t think it would hurt to ask if there are any recommended well documented “catch all” libraries that cover the ’03 to ’10+ excel spectrum? Spending money on a library isn’t an option for me.