Sorry, @DocumentsDir should have been - @MyDocumentsDir
Libraries are ultimately shortcut files with the extension .library-ms files and can be located in @AppDataDir & '\Microsoft\Windows\Libraries.
You can use something like the following and it will open to that location
ShellExecute('Explorer.exe', @AppDataDir & '\Microsoft\Windows\Libraries\Pictures.library-Ms')
The file itself is actually xml, if you use a parser you can grab the <simplelocation>><url>knownfolder:{Guid} you can view a list of Guids and there locations here:
https://msdn.microsoft.com/en-us/library/bb882665(v=vs.110).aspx