Sword of the Rightful King, A Story of King Arthur by Jane Yolen, Magic Carpet Books, Paperback 2004
The newly crowned King Arthur has yet to win the support of the people. Merlin must do something before the king is betrayed, or murdered, or--worst of all--gets married. So Merlin creates a trick: a sword magically placed into a slab of rock that only Arthur can withdraw. Then he lets it be known that whosoever removes the blade will rule all of England, and invites any man who would dare, to try to pull out the sword. But then someone else pulls the sword out first.