Do you have any other evidence that this exists other than the first book mentioning it? Because unfortunately that is not enough to know that it is real. It could have just been a joke, or its publication could have been cancelled after the first book was written, etc.
"Yes it DOES matter," says Mabel Pines, holding two sweaters in her hands. "The sweater I wear has to match the day perfectly! It's sunny, so I feel like wearing my 'Puppy with Shades' sweater. But I've also got a really good feeling about this 'Glittering Yarn Ball' sweater."