The Mysterious Benedict Society by Trenton Lee Stewart (post by the Laundry Queen's daughter, Pen, age 13)

The Mysterious Benedict Society is a book ideal for children anywhere in the age range of 8 years and above. It begins with a precocious orphan by the name of Reynard Muldoon (Reynie for short) glimpsing an ad while perusing the newspaper. The ad seems to be directed to children, and provides a special opportunity for gifted children and adolescents. Reynie decides he fits under this category, and is very curious to figure out what this is about. Miss Perumal, his guardian, takes him to the location specified in the ad, and Reynie finds that the special opportunity happens to be a test. This test is comprised of questions that are very curious indeed. The end of the test comes, and only Reynie advances to the next round. He takes two more tests, the first, a very rigorous and challenging test, comprised of  very little known facts, and the second, an obstacle course. He passes each, and is admitted into a society made up of children, dubbed the Mysterious Benedict Society. He, along with three other children, travel to a house so filled with books it could almost be called a library. In this house, with the aid of Mr. Benedict and his three accomplices, they are informed of the dangerous situation at hand, which they must help to amend. They travel as spies to a special academy, where their most dangerous enemy, along with his devious device, resides as head. At this academy, the children are obligated to save the world, at any cost, using each of their unique skills to their benefit. If they don't prevail, the world will be taken over in a matter of days.
To read more, you can find The Mysterious Benedict Society at any local library or bookstore. It also has two more novels in the series, which are equally fascinating. Good luck and happy reading!
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Pen