I want to read same book like thriller or adventure or fantasy but not too hard, at least level B2
I suggest you read 'The curious incident of the dog in the nighttime' . The language is rather simply even though the story is quite complex.
You can take a look at the PDF version of the book here: http://innerline.chat.ru/curious_dog.pdf
Harry Potter's books are very helpful. if you like this kind of book for sure