Several years ago, a middle-school English teacher named Rick Riordan developed an intriguing concept designed to inspire interest in mythology among his students.

His idea? Imagine yourself as a modern-day kid who is a "half-blood" -- one of your parents is human and the other is a Greek god. Imagine further that these Greek gods are still warring with each other after all these centuries, and that now their battles also are being carried on through their offspring in places such as New York and Los Angeles.

This is the basic storyline for Riordan's hugely popular "Percy Jackson & the Olympians" series. Since "The Lightning Thief," the first book in the series, was published in 2005, Riordan's audience has grown by leaps and bounds, attracted by his fantastical "half-blood" idea as well as by his middle-school humor. Best of all, there's Percy himself, the son of a human mother and Poseidon, the Greek god of the sea. Despite his background, Percy is a normal kid who struggles with attention-deficit disorder and dyslexia as he works to fulfill his apparent destiny to help good defeat evil.

So far, Percy's made it safe, if somewhat battered, through three adventures detailed by Riordan in "The Lightning Thief," "The Sea of Monsters" and "The Titan's Curse." But nothing has prepared Percy for the challenges in the series' just-published fourth installment, "The Battle of the Labyrinth" (Disney-Hyperion, $17.99, ages 8-12).

Now a high school freshman, Percy is hoping to quickly make it through a one-day orientation to his new school before heading off to an afternoon of fun with Annabeth, the daughter of the Greek god Athena, and one of his best friends from Camp Half-Blood. As usual, things don't go as planned, as Percy makes plain in the book's opening sentence: "The last thing I wanted to do on my summer break was blow up another school." Things quickly go downhill from there, and the situation gets really serious when Percy and Annabeth discover that Camp Half-Blood is imperiled by a secret entrance into the dreaded Labyrinth created by Daedalus.

In an effort to prevent the evil Kronos from resurrecting his body and sparking a war, Annabeth and Percy -- along with Tyson, who is Percy's Cyclops half-brother, and Grover, their satyr friend -- head into the Labyrinth, a maze so devilishly tricky that many people lose their minds before they can figure out how to get out. It's the most difficult task Percy ever has faced, and one that will test the limits of his intelligence, physical stamina and, yes, sense of humor.

Once again, Riordan has written a book that will have readers laughing out loud as they race through the pages. It's a potent mix of humor and drama that has built a worldwide following for the series, leading Disney-Hyperion to do a first printing of 1 million copies of "Labyrinth."

Readers don't have to begin at the beginning of the series to enjoy "Labyrinth," as Riordan carefully weaves in background information about the characters throughout the text. But those who have read the series from the start will particularly enjoy this latest adventure.

Similarly, you don't have to know anything about Greek mythology to follow Percy's story. Yet, readers who do know the basics of Greek mythology will be better able to enjoy Riordan's sly humor and the way he takes the stories of the gods and reworks them into modern-day equivalents.

In "Labyrinth," for example, Percy and the others meet what seems to be a New York City doorman, except that he has two faces. It's actually Janus, a minor Greek god who attempts to confuse their efforts to find their way through the Labyrinth and possibly lead them to their deaths.

Then there's the Sphinx, who has replaced riddles with standardized test questions. Riordan raises an issue that every test-weary public-school student will immediately recognize when Percy and Annabeth destroy the Sphinx's automatic grading machine, prompting the Sphinx to scream: "My grading machine! ... I can't be exemplary without my test scores!"

While "Labyrinth" eventually comes to a conclusion, it's also clear that Percy's work isn't yet done. Riordan himself has said he plans one more book in the series before moving onto a new fantasy series.

Meanwhile, readers can enjoy what just may be the best book in the series so far.