The last in the series is called Sonic in Castle Robotnik. In the photo a traditional "Eggman" version seems to be holding a drill and a springy cord. Your screen does not decieve you, this photo really is blurry. Gear needs a better one!
"It's the FrankenSonic monster! In this, the fourth book in the series, Doctor Robotnik plots to defeat that pesky hedgehog once and for all - by building a bolt-together version of Sonic.Werewolves, vampires and mummies lie in wait for Sonic and his sidekick Tails as they venture into Robotnik's mountaintop castle for a cataclysmic confrontation."