story identification - Fantasy book about a boy who crashes into a world

If my memory serves correctly, it was something along the lines of this: ->

1. Boy hurtles into the world, crashing and losing his memories as a result.



2. He's a pirate? Somebody on a floating city takes him in, and he serves as an assistant.


3. 
   

   He goes north to this forest that's incredibly dangerous at night and goes to a slave market and fights in a gladiator ring.

   
   


4. 
   

   He escapes and falls down this hole into the people-group's disposal system, which is like a giant everything-eating organism (excuse my phrasing, please)

   
   


5. The book had illustrations. Black-and-white ones, and was in a series. This may have been the third or fourth book. I think the binding was tan, but I'm not too sure.


6. Miscellaneous details: There's a floating/or on a mountain academy-city; man-eating snails; giant, man-eating forests at night; a generally dark tone; swamps?; most likely published in the late 90's ('98 and up) and early 2000's, because I remember reading this in 2005 or 2006.