The Son of Neptune by Rick Riordan (Heroes of Olympus, Book 2)

Percy is back! Well, back is a strong word...more like less lost? He can't remember anything from before the wolves found him...except for Annabeth. He's pretty sure she's his girlfriend. At least Lupa and her wolves found him, and put him on the path to Camp Jupiter. He almost doesn't make it to their front door! After being chased relentlessly by some gorgons who just *refuse* to die, he manages to make it to the camp relatively unharmed, with the help of some new friends: Frank and Hazel.

Once inside, he learns that all demigods are found by Lupa, trained, and sent to Camp Jupiter for 10 years of service fighting monsters and defending the camp. After that, many choose to enter the real world, or move into the city, where they can grow up and build families in the relative safety of Rome. (Wait...demigods can grow up? Something about that seems odd to Percy.)

He is assigned to the 12 Legion (aptly named Lightning), 5th Regiment. They aren't exactly known for their winning streak, but things get a little crazy after dinner at War Games. Frank comes up with a plan that just might give them the upper hand! It even gets him noticed by his dad. After a personal visit from a god, the three are charged with a quest to stop Gaea and her sons the Giants.They must act quickly, because Gaea's army is already approaching Camp Jupiter. If they can't return successful in three days, the entire city is doomed.

Along the way, Percy starts to get bits and pieces of his memory back. Some are bigger than others, and leave him in less than fighting condition. Without Frank and Hazel, there's no telling what could have happened. The three work together, stopping for help along the way. Frank must discover what his family secret is if they are ever to stop the Giants, let alone Gaea. They run into Isis, the Amazons, some Laestrygonians, and much worse...will they ever defeat Alcyoneus and rescue Thanatos?

“Um...is that thing tame?" Frank said.
The horse whinnied angrily.
"I don't think so," Percy guessed. "He just said, 'I will trample you to death, silly Chinese Canadian baby man',” (Riordan, 2011).

*Library Link*

If you liked this, check out:
Mark of Athena by Rick Riordan (Heroes of Olympus, Book 3) Fall 2012
Eon by Alison Goodman (Eon, Book 1)
The Power of 4 by Pittacus Lore (Lorian Legacy, Book 1)
The Ruins of Gorlan by (Ranger's Apprentice, Book 1)

Riordan, Rick. (2011). The Son of Neptune. New York, NY: Disney/Hyperion Books.