Code Name Bananas by David Walliams. Ages 8+.
Code Name Bananas (2020) is set during the London Blitz in World War Two. Eleven-year-old Eric lives with his formidable Great Aunt Dotty, who works at London Zoo. When the zoo is bombed and a young gorilla named Gertrude is separated from her enclosure, Eric and Dotty must hide her in their flat. Meanwhile, a German spy ring is operating in the neighbourhood.
The novel combines slapstick comedy (a gorilla loose in a London flat), genuine wartime atmosphere and an exciting spy plot. Eric discovers unexpected bravery as the story develops.