The Staff of Serapis

In this adventure, Annabeth encounters more oddities in the subway than usual, including a two-headed monster and a younger blond girl who reminds her a little of herself. This is the story fans have asked for, in which Annabeth Chase teams up with Sadie Kane. The demigod daughter of Athena and the young magician from Brooklyn House take on a larger-than-life foe from the ancient world. Perhaps even more disturbing than the power-hungry god they encounter is the revelation that he is being controlled by someone someone all too familiar to Sadie. As a special bonus, the first chapter of THE BLOOD OF OLYMPUS, the final book in the Heroes of Olympus series, is included.
"UNTIL SHE SPOTTED the two-headed monster, Annabeth didn’t think her day could get any worse. She’d spent all morning doing catch-up work for school. (Skipping classes on a regular basis to save the world from monsters and rogue Greek gods was seriously messing up her GPA.) Then she’d turned down a movie with her boyfriend, Percy, and some of their friends so she could try out for a summer internship at a local architecture firm. Unfortunately, her brain had been mush. She was pretty sure she’d flubbed the interview. Finally, around four in the afternoon, she’d trudged through Washington Square Park on her way to the subway station and stepped in a fresh pile of cow manure."