A book nook or a bookshelf insert is a miniature diorama designed to fit between books on a shelf. They often depict a form of a street or similar setting in a theme from famous books. Book nook can also make a form of a 3D model. They are similar to bookends but are placed in the middle of the books rather than on the ends.
The idea seems to have originated in 2018 from a Japanese artist called Monde. The concept became popular on Reddit, with the r/booknooks subreddit going from zero to 7,900 members in just two days.
The model is usually inspired by a theme from a famous book, e.g. Harry Potter (like popular Diagon Alley). People can buy book nooks fully assembled, or create them on their own. There are book nook kits that enthusiasts can use as a starting point.