This can't be the end!
I zoomed through The Last Olympian, the last in the Percy Jackson series (so far.) My daughter was mad because I got it at the library and started in on it before she could! (She's deep in it at the moment, hasn't moved since she started reading it this afternoon...) I did really like it. Our favorite characters are back in the battle- this time to save Olympus (which of course is on top of the Empire State Building in NYC.) I don't want to give too much away, but let's say it does have a fairly happy ending and it leaves you wanting more of Camp Half Blood's adventures!
By the way...
Out of all the books in the series my daughter and I agree that we liked The Sea of Monsters the best.