Question: The Heroes of Olympus is a pentalogy of adventure, mythological fiction and fantasy novels by American author Rick Riordan that collectively form a sequel series to the Percy Jackson & the Olympians series, a pentalogy of adventure and mythological fiction books written by which American author ?
Sentence (from "The Heroes of Olympus"): Riordan introduces Roman mythology into his sequel series as well as several new characters.
Relevant evidence: No