Does Percy and the Olympian have a fifth book?

No, there is no fifth book in the Percy Jackson & the Olympians series. The series consists of five books:

1. The Lightning Thief

2. The Sea of Monsters

3. The Titan's Curse

4. The Battle of the Labyrinth

5. The Last Olympian

There are other books in the Percy Jackson universe, but they are part of the Heroes of Olympus series, which is a separate series that follows a new group of demigods.