The Sonic Storybook series is a collection of Sonic games exclusive to 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.
The "Sonic Storybook Series" name was only used on the American version of the cover for Sonic and the Black Knight.
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 including the old west, a haunted house etc. Greek mythology won the poll. However after the release of the poll, nothing has been announced about the Storybook series since.