So, how long should a nonfiction book be? The average nonfiction book in self-help, how-to, or travel runs from 40,000 to 50,000 words.
The memoir genre tends to be from 60,000 to 90,000 words
Historical texts or biographies range from 60,000 to 200,000 words#
As these vary widely – and writing long texts is a LOT of work – make sure you check with either your desired publisher or desired genre on Amazon to find out the appropriate length for your book. According to Mark Coker of Smashwords, the top 100 bestselling ebooks at Smashwords (sales could be from other sites) averaged 115,000 words. Largely, this means fiction.