Tom's Midnight Garden by Philippa Pearce

 When Tom is sent to stay at his aunt and uncle's house for the summer, he resigns himself to endless weeks of boredom. Lying awake at night, he hears the old grandfather clock downstairs strike .

. . eleven .

. . twelve .

. . thirteen .

. . Thirteen! How strange! When Tom gets up to investigate, he discovers a magical garden.

A garden that everyone told him doesn't exist. A garden that only he can enter . .

. In this enchanted thirteenth hour, the garden comes alive-but Tom is never sure whether the children he meets there are real or ghosts . .