ぽっぺん先生の日曜日 (Poppen sensei no nichiyoubi, Dr. Poppen's Sunday, 1973) is a children's book by FUNAKAZI Yoshihiko (舟崎克彦, 1945-2015). It's a comical fantasy story in the tradition of Alice in Wonderland, but with an adult protagonist, the biology lecture Poppen, who gets trapped in a riddle book and must go through all the pages to find his way back to the outside world.
Children's stories with adult protagonists are rare; and when they do occur, the adult characters are mostly young attractive figures with which a child might want to identify. Poppen wears slippers everywhere, having gotten into the habit after a foot injury. He goes through this story dressed in a walking club track suit. He's not especially brilliant or courageous. On the other hand, he manages to find his way through the world he has been thrown into well enough. When he is first transported into the book, he deduces that that is what has happened immediately, and generally manages to keep his equanimity throughout the subsequent adventures.
Many of the characters he meets are animals, but their behaviour is often human, or at least different from what Poppen's knowledge of biology might tell him. Still his background gives a particular flavour to the narration, with a more informed description of the plants and animals Poppen sees. A few chapters offer non animal protagonists, notably one in which Poppen finds his way about a town populated by cast off clothes which go about performing the roles (policeman, sweetcorn seller etc.) that their wearers would, if they existed.
To progress through the book he must solve each page's riddle, and before that he must find out what the riddle is. This makes the book a little like Alice Through the Looking Glass with its chess move progression. The first chapters establish the narrative formula; but later chapters, especially towards the end throw in unexpected variations on the pattern.
The level of danger and feelings of threat in the story are generally fairly mild; and the tone is generally humorous, with comedy coming from the absurdity of the fantasy world or from mild satire of Poppen's own life outside it.