It's a book i read a few years ago.
It's a puzzle story where a guy and a girl(on opposite sides of the country) got transported to a world of magic. The story starts off with a band drifting down a river playing a song towards a waterfall.
The boy(main character) is being tracked by some assassin type person. Disliked/hated by everyone in the towns.
*Might be the same character as the assassin, not sure*
A guy that can have his body cut up and such and can reattach his body when he likes.
At the end of the first book, the boy was forced to go home and brought with him the hand of this guy, which could still be controlled and felt.
He broke him out of prison of the super powerful mage king, who he was supposed to be the loyal subordinate of.
The female that also got brought to the world stayed in that world while he got sent back to his own world.
He eventually got back to that world, don't remember how.
One of the puzzles that he was given brought him to a seaside cliff in which he met a guy and when he tried to leave was attacked by some giant killer crab blocking his way back from the seaside cliff.
The puzzle was to find a word that could kill the all powerful king, but ended up being a word that the king created as a test for someone to become his apprentice.
i'm hoping this is enough information to get someone to remember the books name. It's on the tip of my tongue but i just can't seem to remember the name of it. wanted to look into it again. If no one can remember it, i'll just head to my local library when i get the chance. It's where i found the book to begin with.