[adamgordonbell]

It varies by book genre, but there are clearly page number requirements for each sub genre.

A history book will not be best selling if its not a giant doorstop of a book. A narrative non-fiction book will be shorter and snappier because different expectations hold.

A fantasy book should be a tome, etc etc.

For non-fiction, editors very much do enforce these limits. A short history book won't be taken seriously, unless its repackaged as a different type of book.