'Grimm Reality', written by Simon Bucher-Jones and Kelly Hale, features the Eighth Doctor, Fitz Kreiner, and Anji Kapoor.
Back Cover Blurb
There is a world where wishes can come true. Where any simpleton can become a king and any scullery maid might be a princess in disguise. Kindness and virtue are rewarded, and the wicked are made to dance in red-hot shoes until they die. But a witch's oven will cook both the virtuous and the wicked alike, and man a frog prince is crushed beneath the wheels of a cart before he gets that magic kiss.
This world has its own rules and it doesn't care that a certain Doctor Know-All and his friends don't know them.
Now other outsiders have come to the world - traders from the stars seeking the treasures that fell from the rip in the sky. There are riddles to be solved, contests to win, flax to spin. The world to survive.
But the World of Wishes is itself in danger from a race of beings with only one wish. And there is a Princess asleep, and a beast awake - and Giants.