When I was prepping to publish my ebooks, I wondered about the proper length, and so did some research. I subsequently included my findings in an ebook how-to guide:
"How lengthy does your manuscript have to be to qualify as a book in general? Or, what are the usual publishing editor's or reader's expectations in size/length? I've seen various sources offer up these word total ranges of acceptability:
(Keep in mind when viewing the word counts below that 75,000 words make about 300 pages, 100,000 words, 400)
+ Under 10,000 is a short story
+ 7,500 through 25,000 is a novelette
+ 24,000 should be a minimum for non-fiction works
+ 20,000 through 50,000 is a novella or short non-fiction book; or under 60,000 a novella; some think novellas are the optimum size for ebook novels today
+ 50,000 through 110,00, or 75,000 through 120,000 is a novel; some say 50,000 is the magic number separating a novella from a novel
+ Some think 60,000 through 80,000 is the current optimal ebook size
+ 110,000 plus is an epic (and likely requires being spread out over multiple volumes)
+ 120,000 and higher is excessive for novels from anyone but already well-known authors
+ Non-fiction works are usually shorter than fiction
+ Modern non-fiction is more visual than in the past
+ Modern ebook novels may not need to be as long as old-fashioned hard copy novels "
-- How to make your own Amazon Kindle ebook in just hours CHEAT SHEET
http://www.jmooneyham.com/cheat-sheet-how-to-make-your-own-amazon-kindle-ebook-in-hours.html
As for my own books, I think I have one or two that are just a little short of 80K, and if any go over 120K, it's not by much. For after doing the research I split very large stories into multiple books.