The White Bull
The story is based on the Greek mythological tale of Europa and the bull, where the white bull is the king of the Greek gods, Zeus, in disguise. Zeus, the sky and thunder god of the Olympic pantheon, often wore a disguise when romantically pursuing mortal women, so as to avoid the eye of his wife and sister, Hera.
The Trials of Apollo
The series follows the trials of the god Apollo, who has been turned into a human as punishment from his father and king of the gods Zeus. Zeus is angry with Apollo for a variety of reasons, most notably for the younger god's allowing his Roman descendant Octavian to rise to power during "The Heroes of Olympus" series by giving him his blessing. The final book of "The Heroes of Olympus", "The Blood of Olympus", takes place about six months before the beginning of "The Trials of Apollo".