I have worked with this API on a project and it is very easy to use and would recommend it if you want to create no fuss spreadsheets.
The functionality available to you is a good range, the ratings are good and the contributors and authors regularly release updates and new builds to indicate a thriving development process.
Where to get EPPlus?
NuGet Site – https://www.nuget.org/packages/EPPlus/
NuGet Command – Install-Package EPPlus
Package and Documentation: http://epplus.codeplex.com/