LibXL

LibXL is a powerful library that, once included in your projects, makes it possible for you to write and read Excel files without requiring the presence of the Microsoft-vetted application, or any .NET Framework for that matter, on the host system.
More to the point, the library can be used to create new Excel projects from square one and, as expected, to extract and edit data from already existing spreadsheets.

Comes with support for some of the most popular programming languages and for all Excel-related formats

Probably the first thing you should know about LibXL is the fact that it is designed to work with all Excel related formats, such as XLS (used by Excel 97-2003) and XML, XLSX, XLSM formats (used by Excel 2007-2013).
One of the most important characteristics of this library is that it comes with support for a broad range of programming languages. For example, it is entirely compatible with C, C++, C#, Delphi, Fortran and many others.

Offers a surprising amount of customization options

It is also noteworthy that this library also comes with support for Unicode and 64-bit platforms. Since we are on the subject, we might as well point out that it is more than capable of writing up to 240000 for in binary Excel-specific format.
Furthermore, it provides you with a decent amount of formatting options. You can change the looks and feel by customizing the alignments, borders, colors, fill patterns, fonts and even cell merging actions.

Well-documented and accessible Excel library for your commercial apps

Last but not least, you might want to take the time and do a bit of reading regarding other, more advanced features. You can start by reading the hefty documentation section on the library's official website and even check out some of the provided examples or demo files.
All in all, LibXL is a powerful and somewhat customizable Excel library that, thanks to its thorough documentation and its lightweight nature, can be easily integrated within you commercial projects and applications.