I load books to my Astak by using Save to Disk with the following expression:
{author_sort}/{series}/{series_index} - {title}
Go to Preferences, Add/Save and you'll see where you can modify the expressions.
I don't have any multi-author series and prefer to organize by author and then title. Series_index puts the series number at the beginning of the title. If there's no series, then it's just an author folder and the book. Calibre is smart enough to realize when that author already has a folder and saves new books to that author's folder.
Taking out the author_sort expression should give you what you want for a multi-author series, but I think you'd have to change expressions if you wanted the author sometimes included and other times not. Fortunately, the drop down remembers past expressions that you've used, so you don't have to remember what you've used before.
Unfortunately, you have to go back in to Preferences each time to change it. I'd like to be able to save by tags sometimes (unread, for instance), but can't have tags as part of the above expression without creating havoc, especially when the book has multiple tags.
It would be nice if the Save To dialogue box had Save by Series and Save by Tags as options (please Kovid?
), especially if the dialogue let you specify which tag to use.