The Sea of Monsters by Rick Riordan

Monsters never die. They are reborn from the chaos and barbarism that is always bubbling underneath civilization, the very stuff that makes Kronos stronger. They must be defeated again and again, kept at bay. Heroes embody that struggle. You fight the battles humanity must win, every generation, in order to stay human.
-- Chiron speaking to Percy

Percy Jackson once again finds himself in the middle of a dangerous adventure, in the second book of the Percy Jackson & the Olympians series. In the first book, Percy is introduced to Camp Half-Blood, a safe place for demigods -- the offspring of Greek gods and humans. Some kids go there during the the summer, and some stay there all year long. Sea of Monsters opens with the camp coming under attack -- somehow its magical borders have been breached -- Grover in danger, and Percy getting kicked out of yet another school.

Percy and Annabeth set out to find and rescue Grover, who is being held by the cyclops Polyphemus (the same cyclops that Odysseus blinded). They are also trying to find some way to restore the tree that protects the boundaries of the camp. They are helped in their quest by Tyson, a young cyclops that Percy befriends and later finds out is his half-brother. Cyclops are are the offspring of the gods and nereids. Clarisse, a daughter of Ares and fellow Camp Half-Blood resident, also shows up throughout the book, sometimes helping, sometimes causing problems. What do you expect from a daughter of the god of war?

Most of the action in the Harry Potter books happened during the school year at Hogwarts. The Percy Jackson books are the exact opposite. The school year is almost completely ignored and the books begins with Percy heading to Camp Half-Blood for the summer. I point out some other similarities between these two series (Riordan wrote his much later than Rowling) in my review of the first book, The Lightning Thief.

If you approach these books with the right mindset, they are a lot of fun.