It did seem a bit weird (not to mention pretentious) to find that 'Phantom of the Opera' had been filed under 'F' for 'Fantôme' in the Books & Literature category, while being nowhere to be found under 'PH' (which goes directly from 'Phantom of Manhattan' to 'The Phantom Ship') -- that was one reason it took me so long to locate anything! But if they're just typing 'Phantom of the Opera' and having it auto-corrected, that explains a lot...

The information about searching for multiple tags is in the last paragraph of the faq page What are tags?

In my defence, it looks as if that *may* be a new feature post-dating my last FAQ search in quest of such a thing -- it's a note at the bottom of a page that says it was updated as of 2018, while I was being frustrated by the inability to search for intersecting tags back in August 2017!