Hermione might give it to Harry. She is always researching and stumbling upon strange and wonderful magical items. She could have discovered the 'tr' in the library or during one of her side - quests for knowledge. Knowing Harry's importance in the battle against evil, she gives it to him, believing that he will be able to figure out its uses better than anyone else.
Well, it might have a wide variety of stories. Each volume could contain different types of children's tales, like fairy tales, adventure stories or moral stories.
Perhaps 'tr' is a time - turning relic. Harry Potter gets it and is faced with the moral dilemma of whether to use it to change past events. He might consider using it to save his parents, but then realizes the potential consequences of messing with time. This could lead to a whole adventure where he has to make difficult decisions and also face the Time - Turner's side - effects, like being in two places at once in a confusing way.
Stephen King's stories in TR90 are typically filled with elements of horror, suspense, and mystery. They often have complex characters and unexpected plot twists.