http://www.yes24.com/Product/Goods/32286209Another Korean book, with a similar format to the previous one I posted, but the art is better and more realistic. An alien girl (who, beside from the pointy ears and bushy eyebrows, looks exactly like a normal human girl) comes to Earth to find a way to cure her sick mother, and crashes into the protagonist boy, who was bringing his mother's gift to his young uncle/teacher(?), a doctor. Halfway through the book, while they were having lunch, the uncle got a small fish bone lodged inside his mouth. The kids shrink the alien ship down and go inside his mouth, but are accidentally swallowed after having removed the bone from the uncle's mouth. They escape later when they go inside his bladder and are peed out. Later, the uncle trips and injured his head. The kids shrink the ship down again and go inside his bloodstream through the wound. They go to his brain to get energy (?) for the ship and get out through the nose, where the uncle sneezed them out.
Here's PDF version for you lovely people:
https://www.scribd.com/document/413371632/BritannicaI've skipped the part where they are not inside his body, as well as the part where they study about senses, as there aren't any good internal shots, and most of them are just dialogues explaining stuff.