Gets the string representation of this workbook reference.

The returned string depends on how the reference was created. If this reference has no workbook name, the returned string is the workbook ID enclosed in square brackets. If it uses a base URI, the returned string combines the unescaped base URI with the unescaped workbook name in square brackets. If it stores only a workbook name, the returned string is the name enclosed in square brackets. If it uses a base directory, the returned string combines the base directory and the workbook name in square brackets.