If you loved the Percy Jackson & the Olympian series, then try these other great action-packed adventure series.
The Alchemyst: Secrets of the immortal Nicholas Flamel by Michael Scott (no, not that Michael Scott)
While working at pleasant but mundane summer jobs in San Francisco, fifteen-year-old twins, Sophie and Josh, suddenly find themselves caught up in the deadly, centuries-old struggle between rival alchemists, Nicholas Flamel and John Dee, over the possession of an ancient and powerful book holding the secret formulas for alchemy and everlasting life.
Wolf Brother: Chronicles of Ancient Darkness by Michelle Paver
6,000 years in the past, twelve-year-old Tarak and his guide, a wolf cub, set out on a dangerous journey to fulfill an oath the boy made to his dying father--to travel to the Mountain of the World Spirit seeking a way to destroy a demon-possessed bear that threatens all the clans.
Jack Blank and the Imagine Nation by Matt Myklusch
Twelve-year-old Jack, freed from a dismal orphanage, makes his way to the elusive and impossible Imagine Nation, where a mentor saves him from dissection and trains him to use his superpower, despite the virus he carries that makes him a threat.
Passenger by Jane Yolen
Abandoned in the woods of medieval England, a young boy grows wild during the subsequent year in which he is forced to forage for food and run from wolves, until he is rescued by a falconer and remembers his name and the legacy of magic he is to inherit.