This is book 2 in the Percy Jackson series and it is a great read.
You definitely need to read book one
In this second adventure in Percy Jackson and the Olympians series, Percy is anticipating the end of seventh grade and a summer at Camp Half-Blood with kids who are the offspring of Greek gods and mortals. He dreams that his pal Grover, a satyr, is in danger. After monsters attack in his school gym class, Percy and Tyson, a homeless kid, are picked up by Annabeth, a half-blood friend, and rushed to the camp, which is under attack. The tree that guards the camp is dying, and Chiron, the activity director, has been dismissed. Another dream reveals that Grover, whose peril is increasing, is on the same island as the legendary Golden Fleece, which may be the cure for the troubled camp. With the help of Hermes, Percy and his pals set off to the Sea of Monsters, where they encounter legendary dangers. Riordan’s clever mix of classical mythology, contemporary teen characters, and an action-packed adventure will bring new readers to the series.
I recommend it (as a series) 4 out of 5 stars