The Edge Chronicles Volume 4: Curse of the Gloamglozer by Paul Stewart and Chris Riddell
This fourth volume of The Edge Chronicles jumps ship, as far as timeline and characters are concerned. A trip to the past helps deepen the story by giving us more information that will have a bearing on future portions of the story.
While I suppose you could start with this book, that's an almost unthinkable concept for me. However, if you haven't checked out this series yet, I would highly recommend it - as would Rachel and Justin. The world is highly detailed and lifelike. The ecology of the fantastic creatures - of which the Gloamglozer is central - has levels of structure; although some things seem whimsical or horrible, there is actually reasoning behind why the evolved to do what they do. Or are.
If you've started reading this series already, you become familiar with the Gloamglozer in the first book - he's what mothers use to discipline children, the bogey monster, the devil, and more. The stories don't pretend to be more than a fun adventure, but, unfortunately, they are. There's quite a bit of depth if you want to scratch under the surface. Some of the greatest themes in fantastic literature are handled here. There's echoes of Frankenstein, glimpses of Gormenghast, and more.
But then, it's just a kids book. Fun for all ages, eh?