The Immortal Secrets of Nicholas Flamel series by Michael Scott
This ongoing series is about the twins of legend, Sophie and Josh, and their adventures trying to save the world from Macchiavelli, Dr. John Dee, and Billy the Kid. To aid them in this quest are St. Joan of Arc, Nicholas Flamel, Sophie Perenelle, and other figures from history and myth. The first title is The Alchemyst, followed by The Magician, The Sorceress, and The Necromancer with more to come! Highly Recommended.
The Red Pyramid by Rick Riordan
Yes, it's the same formula as The Lightning Thief series but with twins raised apart, a boy and a girl, whose father is a noted Egyptian archaeologist. When their father becomes trapped in an obelisk, the twins step in to find a way to free him. Of course, they encounter many ancient Egyptian gods, shabtis, and magicians, both helpful and nefarious. I, personally, liked The Red Pyramid better than The Lightning Thief. Highly Recommended.
Century Book #1: Ring of Fire by Perdomenico Baccalario
Four children, from all over the world, find themselves in the same hotel on the same night in Rome, Italy, when they discover they have the same birthday, February 29. As Random House says, "Every hundred years, four kids from four cities must save the world." Highly Recommended.
The Sea of Trolls by Nancy Farmer
I read this book last summer, and really enjoyed the plot twist. Two children are captured by Viking Berserkers and taken to the home of King Ivar the Boneless and his half-troll queen. This book was a little gruesome for the younger reader. It's more appropriate for 7th grade and up. Highly Recommended for age 12 and up.
The Seven Keys of Balabad by Paul Haven
Set in modern times, Balabad is a country very similar to Afghanistan +or Iraq. Oliver Finch is a New Yorker, whose journalist father and archaeologist mother are working in Balabad for a year. He has made a total of two friends: Mr. Haji, a carpet salesman in the marketplace and the Zee, a very, wealthy and lonely native boy his own age. When Mr. Haji disappears under mysterious circumstances, Oliver and Zee go on a rescue mission. This one is my favorite of this list! Highly Recommended.
The Kingdom Keepers: Disney After Dark by Ridley Scott Pearson
A group of teens are hired to be "hologram" hosts for Walt Disney World. Something goes wrong, when they begin to find their consciousnesses transported into the Magic Kingdom at night. They band together to fight Maleficent and a crew of Disney villains who want to destroy Mickey and take over Walt Disney World. There are two other titles in this series: Disney at Dawn and Disney in Shadow. I enjoyed these, but I am a Disney fanatic. The writing, plots, characters are not the grade A caliber of the rest of this list, but they are fun. Recommended.
The Mysterious Benedict Society by Trenton Lee Stewart
Chosen by Mr. Benedict, four extremely gifted children are sent on a secret mission undercover at the Learning Institute for the Very Enlightened. Other titles in this series are The Mysterious Benedict Society and the Perilous Journey and The Mysterious Benedict Society and the Prisoner's Dilemma. Excellent mental exercises abound in these books. Highly Recommended.
And don't forget to check out Borders and Barnes & Noble summer reading programs! Get a free book at each.