I am always on the lookout for some high interest series books. It seems like Junie B Jones and Magic Tree House have been read and re-read by every student at my school. While I admit that I enjoy series books, I can't imagine not branching out a bit more, or at the very least, exploring new series.
Secret Agent Jack Stalwart is a new hero that I am hopeful my students will soon be fighting over. The first book in the series, The Escape of the Deadly Dinosaur, is full of suspense that I think readers will find appealing. Jack is a secret agent assigned to rid the world of evil. He receives his directions and then will end up being carried off to the location of his job. He posesses a few secret agent tricks that help him in his endeavors. Each book takes place in a different location and there is a bit of information about the locale included, without becoming too school-like. Kids might actually be picking up little facts without even realizing it.
These books are right around one hundred pages in length with short chapters, and are quite comparable to the Magic Tree House series. Written at a bit higher reading level, they would be a good fit for those readers who need to challenge themselves a bit more, but don't have the attention span or stick-to-it-iveness needed for longer chapter books.