The Merlin Conspiracy by Diana Wynne Jones

When the Merlin of Blest dies, everyone thinks it's a natural death. But Roddy and Grundo, two children traveling with the Royal Court, soon discover the truth. The Merlin's replacement and other courtiers are scheming to steal the magic of Blest for their own purposes.

Roddy enlists the help of Nick, a boy from another world, and the three turn to their own impressive powers. The dangers are great, and if Roddy, Grundo, and Nick cannot stop the conspirators, the results will be more dreadful than they could possibly imagine.