The Sonic Storybook series is a collection of Sonic games exclusive for the Wii, in which Sonic the Hedgehog travels into the world of a storybook to stop a great evil that potentially threatens both his world and theirs, meeting alternate versions of his friends and enemies in the process.
It is never actually explained what method Sonic uses to return to his home world after defeating the evil in either storybook game. Though, it can be assumed he left the same way he came, with the magic of Shahra (in Secret Rings) and the summoning spell of Merlina (in Black Knight).
On the back of both boxarts, Sonic is in the same pose.
Tails and Knuckles are the only characters besides Sonic that appear in all of the Storybook Series games so far, as alternate forms.
Sega ran a contest to see what the fans wanted Sonic to go to in the third installment of the Sonic Storybook series with places like the old west, a haunted house etc. Greek mythology and myths won the poll.
However, nothing has really come out of this so far, and the poor sales of the second game before it make a third unlikely.