The Flat Man by Rose Impey, 1988.
Before Slender Man, there was . . . The Flat Man! And he wasn’t quite as scary. As far as I know, there is no connection between the two fictional characters other than a similarity in name. Still, the Creepies Series, while picture books, should probably only be given to children no younger than seven who like scary stories. They might provoke nightmares in very young children and sensitive children. I liked these stories as a kid for their imagination, but individual tastes may vary.
Like all the books in the Creepies series, The Flat Man features a child who enjoys being a little scared and makes up his own imaginary monster to battle with at bedtime. The entire time, the boy knows that the monster is imaginary and prides himself on knowing just how to deal with him.
While lying in bed one night, the boy imagines the scary creature called the Flat Man for the fun of it. He amuses himself with imagining that every sound he hears is the Flat Man, a paper-thin creature that can sneak in anywhere, squeeze himself through any crack. But, the Flat Man has weaknesses: he’s afraid of light and open spaces. The boy acts out his duel with the imaginary Flat Man, enjoying his triumph . . . right up until his father comes in to find out what’s going on.
Even though his imaginary creature is frightening, the boy enjoys it, comes up with his own solution for defeating it, and acts out a triumphant battle with it. Those who don’t mind a bit of spookiness will appreciate the boy’s imagination and may giggle at how he gets a bit carried away with his fantasy.