Here's why:
* The sentence structure: The sentence is "That novel became an overnight bestseller." The verb "became" is a linking verb, connecting the subject "novel" to a description or state of being.
* The role of "an overnight bestseller": This phrase describes the state of being of the novel after the change. It clarifies what the novel became, acting as a complement to the subject.
Let me know if you'd like to explore other types of complements!