[At_Libitum]

Don't forget that you can make multiple selections in the tag browser using ctrl-click.

e.g. your first example to select an author and then see all series from just this author,
click the author name in the tag browser,
press and hold ctrl and click the series category.

The same method of course also works in selecting a subset of tags.

additional filtering can indeed be done using virtual libraries, there is also the use of hierarchical data as you seem to have found (e.g. I have a separate column for the main genre and combine these with the tags in a separate custom column shown in the tag browser, so I get an entry like Sci-Fi.Speculative or Sci-Fi.Utopian

If this column is then added in the look&feel for the tab browser as being a column with hierarchical data, it will create 'subfolders' in that category in the tag browser.

so the former would end up as
Sci-Fi
|----Speculative
|----Utopian

(note. when clicking a second time on a parent level in hierarchical tags, the single green plus turns to double green plusses, showing every book below that level. And yes, it would be nicer if it did this by default on first click)

Combine this with virtual libraries on for instance the main genre column, have the virtual libraries shown as tabs and you get the beginnings of a very flexible way of filtering, selecting and searching.

Of course such combined columns don't need to be limited to genre.tags but could also be applied to e.g. Series.Authors, this is just how I happen to be using Calibre and I am sure others will have yet other tricks in their toolbox to make this an easier task. Also they don't need to be limited to two levels, you could for instance create a custom column series.authors.genre.tags or whichever order you like for alternative forms of filtering

Also don't forget the Save Search option to save intermediate steps.

e.g. the first example you gave. Once you have selected the author and ctrl-clicked the series category, save this as a custom search "Series by Author X" to have a quick starting point the next time. And if you find you use a specific search a lot you can use the same criteria to make it a virtual library.

Virtual libraries have one other advantage over custom searches and that is inside a virtual library, all of the categories in the tagbrowser will also be filtered and only showing those category entries present in that virtual library.