The Titan's Curse: Percy Jackson and the Olympians, Book 3 (by Rick Riordan)

When his friend Annabeth and the goddess Artemis both disappear just before the winter solstice, and when Artemis must be on Mt. Olympus to convince the gods to prepare for a war with the titans, Percy, his friends, and two of Artemis' followers set out to rescue them, led by a prophecy and Percy's dreams. But Kronos is preparing an elaborate trap for them, and Luke has been gathering an ever larger army.

Rick Riordan shows how formula fiction should be done. Each novel in the series so far follows the same basic path: Percy goes on a quest that usually involves both rescuing someone close to him and at the same time accomplishing some task for the gods. All of these stories are part of an overarching story arc that involves the reawakening of the evil titan Kronos and a coming second apocalyptic battle between the gods and titans, presumably in book 5.

This kind of predictability is appealing to middle-grade readers, who like the comfort factor. So it is up to the author to keep it interesting and fresh. This Riordan does with a now practically trademarked blend of action that seems more violent than it is, witty humor that doesn't pander, and a wealth of hilarious modern takes on classical myths, all told by an appealingly loyal and self-deprecating hero. So far the author has been predictable in one other way as well -- the fun and the quality are both consistently high.

From the Book:
The driver got out, smiling. He looked about seventeen or eighteen, and for a second I had the uneasy feeling it was Luke, my old enemy. This guy had the same sandy hair and outdoorsy good looks. But it wasn't Luke. This guy was taller, with no scar on his face like Luke's. His smile was brighter and more playful (Luke didn't do much more than scowl and sneer these days). The Maserati driver wore jeans and loafers and a sleeveless T-shirt.

"Wow," Thalia muttered. "Apollo is hot."

"He's the sun god," I said.

"That's not what I meant."