Firstly, apologies for my poor artwork but hopefully you’ll recognise what I’m writing about today from the accompanying image. Mystic Drawing books were a close relative of the Magic Painting books, except instead of using a paintbrush and water you used a bog standard pencil.
Each page initially appeared blank, but when you rubbed a pencil softly over the page lines would magically appear on the paper. By covering the entire page in this way you eventually revealed a cartoon like picture. You did have to be gentle with your colouring though, as pressing too hard meant the revealed lines and your shading would be too similar and the effect wouldn’t work quite so well.
I’m not exactly sure how Mystic Drawing worked, but the pages were very shiny and if you caught the line on them right you could actually see where the lines would appear when you coloured over it. I think it may have been as simple as the lines being “printed” as a slightly rougher surface, so when your pencil strayed across more graphite was left behind and the line was therefore revealed. The shiny surface of the paper did mean it was kind of difficult to colour in the picture after you had revealed it all, but one option (if you cheated and looked sideways at the page to see the final picture) was to use coloured pencils to fill in different lines in different colours.
I guess the modern equivalent of this would be those mini games you get in some games on the Nintendo DS, where you wiggle the stylus across the screen to reveal a picture or brush dust out of the way. Games such as Legend of Zelda: The Phantom Hourglass, Wario Ware Touched and Another Code spring to mind has having sections which reminded me of the many happy hours I spent completing a Mystic Drawing book.