What is wrong with old fashioned language? The Victorian wrote the most beautiful prose. I guess Oliver twist and Tome Sawyer are out....

Diary of a Wimpy Kid is popular here.
The first harry Potter (Philosopher's stone)
The Series of unfortunate events
The 13/26 etc story treehouse
Roald Dahl (although they have lots of violence)
My older boy loves the Lonely Planet Not For Parents series and he also like Horrible Histories (lots of violence in them).