The commercial productI've just recently used FlexCel.NET isand found it to be an excellent library! I don't say that about too many software products. No point in giving the whole sales pitch here, you can read all the features on their website.
It is a commercial product, but you get the full source if you buy it. So I suppose you could compile it into your assembly if you really wanted to. Otherwise it's just one extra assembly to xcopy - no configuration or installation or anything like that.
I don't think you'll find any way to do this without third-party libraries as .NET framework, obviously, does not have built in support for it and OLE Automation is just a whole world of pain.