Percy Jackson and The Lightning Thief

This book is when Percy is brought to Camp Half-Blood but gets sidetracked and kills a Minotaur. He practices fighting in the sword arena.

Percy Jackson and the Sea of Monsters

Percy, Grover, and Annabeth go on an adventure to the sea of monsters to find the golden fleece. There they find a cyclopes going blind.
They need the fleece to help the magical borders which have been poisined.

Percy Jackson and the Titan's Curse

They have to get luke back to the right side.

Percy Jackson and the Battle of the Labrynth

They must get to the workshop of Daedulus to get Adriane's string, which is the only way they can navigate through the Labrynth without being driven insane.

39 Clues: Maze of Bones

This is about a a sister and a brother. They go to their late grandmother's mansion. In her will she has a game for them to play for the family fortune.