SpreadsheetLight is an open source Open XML spreadsheet .NET library written in C#, and is released under the MIT License. You can create new Open XML spreadsheets, or work with existing Open XML spreadsheets that are compatible with Microsoft Excel 2007/2010/2013 and LibreOffice Calc. No Office/Excel Interop needed. You do not need Microsoft Office/Excel to be installed.
$ dotnet add package SpreadsheetLightNo README available.