The average nonfiction book, if such a thing exists, runs about 50,000 to 75,000 words, but this can vary depending upon category. Biographies, for example, can run 200,000 words. How-tos or self-help usually have 40-50,000 words. The average adult novel runs approximately 100,000 words while those in other categories run 80-120,000 words. Every category will have an average length, including children’s books, young adult, business books, etc.