Simple, custom font. Go look for the real handwriting in the series, it would be appears in drawings and compare. Anyway, I wanna say. The Pusheen artist has made multiple fonts for all of her handlettering elements (probably she's too lazy to handwrite), and she made more fonts than this book writer and unfortunately why don't she become a font artist? Don't believe? Go see yourself, see carefully every same glyph and compare.

Custom.