The Lightning Thief
The book is entitled "Percy Jackson and the Greek Gods" in the U.K. and much of the Commonwealth of Nations. It is generally published under the Puffin Books imprint of Penguin Group and may be presented as Percy Jackson and the Olympians, volume 6. (The Olympians novels are commonly entitled "Percy Jackson and the Lightning Thief" rather than "The Lightning Thief", and so on.)