I read this book around 2011. Its about a trio of characters that get killed very early on in the book and get sent to a world where willpower is the main force. For example, you don't physically walk but have to "will" yourself to move. The characters go to London, where they are investigating someone who murdered the father of one character. They are actually shot and killed by this person in a subway or train carriage. They then get sent to the world and have to survive almost being killed by imps with sharp beaks(?). Average people are actually normally killed by these beings, but for some reason, the characters have enough willpower to move away. This is the first level, eventually they do things like fly etc. I think they eventually have to fight some sort of an entity that is in charge of the razor imps. After this, they reach the upper levels and I think they eventually find large flying fish that have been hollowed out and used as ships.
I can't remember to many other plot details. The cover has a far away view of a green/brown fantasy styled ruined building with stairs going nowhere,sort of like a maze. I think it's meant to be part of a best selling trilogy in the young adult fiction. I have been trying to find this book for many years! Any help would be greatly appreciated.