Are the picture books and the novels different continuities?
The picture books are supposedly abridged, less-detailed versions of the novels, but sometimes, the narrations blatantly contradict each other. For example, "Jack Frost" says Nightlight was transformed into Jack Frost as soon as he awoke from being a Human Popsicle
in the cave with Pitch, but this name is never once used in any of the first 3 novels (the advertising blurb at the back of Toothiana
uses it, implying it would appear in the next book, but it doesn't). It also refers to his ice powers as if he's had them all along, but Nightlight never uses or shows any sign of possessing ice powers at all in the first 4 novels. Nor in any of the first 4 novels are the Guardians shown going to the moon and helping the Man in the Moon break up the dust and rocks to make it shine, like they do in "The Man in the Moon," which the narration describes as if it happened just after he formed the Guardians (at the very least, before Nightlight ever got involved). The passage in "The Man in the Moon" describing what MiM thought the children of Earth needed and choosing people to give them toys, treats, gifts, etc. also acts as if he personally gave each of them their powers and assignments, like he did Sandy, but he didn't — he recruited the others long after they were born with (Bunnymund, Toothiana) or acquired (North) their powers and chose what to do with them on their own.