How can I cite a workbook?
I would treat it as an online resource since it is not published and might be replaced by a newer version at some point. To cite this, you don't need an author but make sure you state the date when you retrieved the resource. In BibTeX it is done using @misc entry for example. 

Another, but less formal, possibility is that instead of a reference, you use a footnote where you will place an URL and the date you retrieved the resource.