Why does he said "50,000 word novel", not "50,000 words novel"?
When a number and unit of measurement are used together as adjectives, even if the unit number or number of objects is more than one, the adjective will be in the singular form. "50,000" is the number and "word" is the measurement unit, which, when added together, become an adjective used to describe the book. Remember that when numbers are used as adjectives in this way, the first two words need to be hyphenated. Ex: 1000-word novel. Ex: Six-foot tall tree. Ex: I worked a ten-hour shift today.