Magyk, by Angie Sage

This is the first book of several in the Septimus Heap series. It takes place in an imaginary world full of witchcraft and mysterious creatures. In it, a boy, Septimus Heap, who is the 7th son of a 7th son and who therefore has strong magical powers, disappears. His father, Silas, is entrusted with the care of Jenna, the daughter of a murdered queen. When Jenna turns 10, evil forces come after her to prevent her from claiming her place on the throne. The book is the story of Jenna's flight from those forces and the people who help her along the way.
The Mysterious Benedict Society by Trenton Lee Stewart

The author of this book got the idea for it while trying to solve a puzzle in the newspaper. It is about a group of kids who answer a strange ad in the paper, the puzzles they have to solve as a result and the reason they have to solve them. Great characters, interesting puzzles and strong plot. The book fair is selling both this book, in paperback, and the hardcover sequel, "The Mysterious Benedict Society and the Perilous Journey"
The Invention of Hugo Cabret by Brian Selznick

My niece considers this to be one of the best books she's ever read. After finishing it, she insisted that her mom read it. Her mom liked it too.
This is a unique book, for is a combination traditional and graphic novel, without being either one. It is the story of Hugo, an orphan who lives in the walls of a train station. Beautifully illustrated with a very intricate plot leading to a fantastic ending. Already considered a masterpiece.
Ender's Game by Orson Scott Card

This is the first book in one of my favorite science fiction series of all time. In it, children are recruited to become soldiers in a war against an alien race. Andrew "Ender" Wiggins becomes a star recruit, but as such, is given increasingly demanding tasks and is kept away from his friends. This is the story not only of his "Battle School" training, but of how, as he is given more and more power and is separated from his friends, he vows not to become evil and maintain his friendships. This book, along with its two sequels, "The Speaker for the Dead" and "The Xenoxide" are true classics and must read books for science fiction fans.