I think your omnibus provides a good clue though. If there is a common theme or something to tie items together then there's no reason the collection couldn't be considered a single book (especially if short). On the other hand all books by an author even in a single file would still be considered a series. And magazines or similar timed publications are still serials no matter how short or long.

Interesting comment about how digital formats figure into it, and I completely agree about the need to realize what the industry is actually based on.