The accidental wizard - Kimberly Pauley

A raucously funny middle grade adventure for fans of
Terry Pratchett and Neil Gaiman...

Twig is the last surviving apprentice of the
great wizard Ripplemintz, which, as a job, is just
as terrifying as it sounds. Oh Ripplemintz always
means well, but for a wizard of such high regard
he really does make an awful lot of mistakes.
And who's always left to clear them up? That's right - Twig.

So when Ripplemitz's most powerful spell is let
loose on the world, off Twig goes to catch it.
And catch it he does, except... not quite in the
way that he intended. Because, instead of catching it in an enchanted
jar, Twig sort of... well... catches it in... HIMSELF.