The Last Olympian by Rick Riordan

Half-blood Percy Jackson (whose father is Poseidon) is nearing the magic age of 16 and has been pondering the prophecy that some bad things would happen on his next birthday. He is still battling Kronos (who has taken over Luke's body) and together with Beckendorf, they have attacked the "Princess Andromeda." Nothing is ever half-done with Percy and right off the bat he is battling a giant (I mean really, really big) crab. Riordan packs this final adventure with action and drama. A good ending to a very good series. Sandy Penton