First, it involves an impressive tourist attraction I'd never heard of before. The London Eye is a gigantic ferris wheel where groups of people ride in sealed capsule-type cars. The kids in the book must figure out how their cousin disappeared while riding in one of those.
The second unusual thing is that it's told in first person by a protagonist who seems to be autistic, although that word is never used. Ted's brilliant mind doesn't work like other people's do, and, with the help of his "normal" sister, he uses his different way of noticing things and thinking about them to solve the mystery. The author did an amazingly good job of helping the reader experience what the world is like from the mind of someone with that condition.
This book would be a good read anyway, but the unique setting and point of view make it even more enjoyable.