The Son of Neptune

The Son of Neptune is the second book in the Heroes of Olympus series written by Rick Riordan, set to be released in Fall 2011, and was revealed on page 557 of the First American Edition Hardback copy of The Lost Hero.

"The Son of Neptune", (Roman equivalent of Poseidon) most likely refers to Percy Jackson, the main protagonist of the Percy Jackson and the Olympians series. At the end of The Lost Hero it reveals that Percy is sent off to the Roman Camp Half-Blood in San Fransisco so they will reveal what Percy is going through as they did with Jason in The Lost Hero. Or it might be a half-brother of Percy (Perseus) Jackson but this is highly unlikely.