You can download the workbook, as defined at http://onlinehelp.tableau.com/v10.0/api/rest_api/en-us/help.htm#REST/rest_api_ref.htm#Download_Workbook%3FTocPath%3DAPI%… But a Tableau workbook is not, and cannot be, an xlsx file. A Tableau workbook is a .twb or .twbx file. The .twb file is the definition of the view in an XML format.

If you mean you want to export the data in the workbook, not the workbook itself, there are a variety of ways, some of which are discussed here:

Excel Export: Two options to massively improve this functionality

The Greatest Tableau Tip EVER: Exporting CSV made simple!

API Reference

I hope that helps.