かがみの孤城 (The solitary castle in the mirror)

Winner of 7 major book prizes in 2017 and 2018, and selling over 500,000 copies, the solitary castle in the mirror tells the story of the first year middle school student Kokoro, and her adventures in the mysterious castle beyond the mirror in her room.
With an intriguing story you won't be able to put down, かがみの孤城 (The solitary castle in the mirror) is the perfect novel for intermediate to advanced learners to improve their Japanese reading skills with.