My 9 year old *loved* The Mysterious Benedict Society books. He enjoyed them so much that he was able to convince the teacher to read the first one as their class read aloud book. There are three books in the series. Great story of mystery and kids having to work together towards a common goal. An interesting aspect that caught my attention is that the kids are faced with a moral dilemma and must decide what to do. Definitely no romance
He also enjoyed Mr. Lemoncello's Library, another series of three books.
An oldie but goodie is The Phantom Tollbooth - especially at this age where they are reading so independently, the puns are great.
I hope you've found some winners!