The Screaming Staircase by Jonathan Stroud
This was such an interesting mix of fantasy, horror and mystery. So good.
The main characters reminded me other famous trios from other authors (Lindgren, Riordan, Rowling) and their relationships with each other made me laugh a lot. I adored George and loved Lucy, but Lockwood was too much of a daredevil for my taste and his annoying habit to keep things from his partners made me angry.
The story was too creepy to be a middle grade book. Hello, the mansion was a horrifying experience and all these other ghosts (and the skull in the jar) ... eerie and amazing at the same time. Of course I couldn't help myself and bought the second book right away.