The City and Its Uncertain Walls by Haruki Murakami

We begin with a nameless young couple: a boy and a girl, teenagers in love. One day, she disappears . . . and her absence haunts him for the rest of his life.
 
Thus begins a search for this lost love that takes the man into middle age and on a journey between the real world and an other world – a mysterious, perhaps imaginary, walled town where unicorns roam, where a Gatekeeper determines who can enter and who must remain behind, and where shadows become untethered from their selves. Listening to his own dreams and premonitions, the man leaves his life in Tokyo behind and ventures to a small mountain town, where he becomes the head librarian, only to learn the mysterious circumstances surrounding the gentleman who had the job before him. As the seasons pass and the man grows more uncertain about the porous boundaries between these two worlds, he meets a strange young boy who helps him to see what he’s been missing all along.