Marvin Terban "Scholastic Dictionary of Idioms"。イディオムの意味はもとより語源まで説明してくれるありがたい本。例えばこんな感じ。
Ace up one's sleeve
(例文) I don't know how Henry is going to get his mom to buy him a bike, but I'm sure he has an ace up his sleeve.
(意味) a surprise or secret advantage, especially something tricky that is kept hidden until needed.
(語源) Back in the 1500s most people didn't have pockets in their clothes, so they kept things hidden in their sleeves. [...] In the 1800s dishonest card players secretly slipped a winnig card, often an ace, up their sleeves and pulled it out to win the game when nobody was looking.