The Invisible Robotnik-
Publisher: Ladybird
ISBN: 0-7214-1739-6
Date: 1994
"The Invisible Robotnik" with "Awesome fold out picture". Which is hopefully not of invisible Robotnik. Ladybird Books had a whole series of these things. They are short stories, with mostly words on illustrated backgrounds. Info by Ota.
Blurb: Sonic the Hedgehog and Ladybird join forced to bring you some fast, furious, frantic and fun-filled radical action!

Summary: Robotnik decides to create an invisibility machine in order to turn himself invisible to stop Sonic. After scaring away the inhabitants of Mobius with his invisible actions, Robotnik realises that Sonic is too much of a match for him to handle even when invisible. All his plans fail and Robotnik ends up returning to visibility and destroying his machine. This story is fully illustrated in colour and takes place in the zones of Sonic 2.