What are the sources for these numbers[?]
These are "best guess" averages by people familiar with the industry. These numbers are based on rough estimates of what a variety of publishers typically seem to want.
[A]re there more I am not aware of[?]
Likely. These estimates tend to break down into two dovetailing categories - basic word counts and genre-specific counts. Per this 2015 blog post:
Length By Type
- Flash Fiction - Under 500 words.
- Short Story - Between 1000 and 8000 words (typical contest lengths). Between 5000 and 10,000 words is a long short story (as for an anthology).
- Novella - Between 10,000 and 40,000 words. Other estimates I have seen typically start at 20,000 for a short novella (perhaps 3-4 in a single volume) and 40,000 for a standalone publication.
- Novel - Over 40,000 words.
"However, very few novels these days are as short as that. Generally a 50,000-word novel would be the minimum word count. Most novels are between 60,000 and 100,000 words. A single novel can be longer, but once the length is above 110,000 words publishers may look at cutting it back, unless it is a particular kind of book – books over the 110K word count are usually considered ‘epics’."
Novel Sub-genre Lengths
- Adult fiction (commercial and literary) - Between 80,000-100,000 words. Books in this genre may fall outside these counts but are generally outliers.
- Science and fantasy fiction
"[E]xceptions to the ‘word-limit’ rule, but even so they don’t usually exceed 150,000 words (and usually fall within the 90,000-120,000 range)."
- Romance novels - Between 50,000-100,000 words.
"…this is a fairly vast bracket thanks to all the sub-genres[.]"
- Historical fiction
"Similar to sci-fi and fantasy-fiction, you are creating a world for your contemporary audience – [...] Aim for the 100,000-word mark[.]"
Suspense Fiction - This category roughly includes crime, mysteries, thrillers and horror.
"Generally speaking a 70,000-90,000-word count is a comfortable range."
Young adult fiction - Between 50,000-80,000 words.
"…however there is a little flexibility here, due to the sub-genres found in YA."
- Non-fiction - Varies greatly with subject matter. "The Diary of Anne Frank" is not going to be the same length as "The ARRL Handbook For Radio Communications".
Westerns weren't mentioned in the above article but most seem to have similar counts to YA fiction (maybe Romance).
Works for children are often between 400 to 1,500 words depending on age of the audience and manuscript type (story book, illustrated story, play, etc.)
[I]s there any current debate on the validity of them?
I kid but honestly those numbers really aren't far off.
You have to remember that publishing is a business - publishers are not doing this as a benefit to authors. These numbers are based on two primary factors:
- What are the production/publishing costs involved?
- What will an audience actually buy and read?
People may well be less likely to shell out to read the next "War and Peace" (which simultaneously costs more to produce and taxes normal attention spans) than "The Pocket Book of Boners".
In most instances, word counts are targets for authors to make so they aren't immediately rejected by publishers. The people working submissions ("the slush pile") use word count as a way to reduce what they have to look at (i.e. by dismissing things that will likely not be viable for publication).
Sticking to a word count
"also shows that you have the ability to pace your narrative and make every word count (that you are disciplined at self-editing)."
In the end, though, these aren't hard and fast numbers in the sense you should have a story length that fits whatever you're writing. But falling outside these guidelines may make it hard to sell your manuscript.
As a secondary benefit, these counts can also help you target whom you send submissions to. If you pick up a Writer's Market book, their publisher listings have submission requirements, often including word count.