Labeling my nonfiction word counts

In the interests of "truth in advertising" I have been trying to find descriptive terms based on word count for my non-fiction work. I figured that fiction has a loose set of terms based on word count, so one should exist for non-fiction. Well, so far I have not find a set of terms. In the lack of anything more definite, I have decided to use the following terms for the shorter pieces of non-fiction.

Short article: less than 500 words
Article: 500 to 2500 words
Long article: 2500 to 5000 words

As for the longer pieces, I still have no clue what to use. For instance, I cannot figure out at what word count a non-fiction piece properly becomes a book.

I do know some fuzzy facts, such as a standard 192-page paperbook contains from 40,000 to 50,000 words, and that book size in the print market was often determined by the nearest hundredth page marker, and print publishers were concerned with spine size (which is why you could not write a hundred page book in the print market).

I have thought about doing an estimated page count, but given the fact that word count per page on printed books vary (standard range is 200 to 350 words per page), I am not sure that will work for my ebooks either.