Which figure of speech is used in this text?
Ronald remarked that the new book on anti-gravity was impossible to put down.

 - pun
 - alliteration
The text uses a pun, a word or phrase that humorously suggests more than one meaning.
Impossible to put down means that the book is so good that it is hard to stop reading. The phrase impossible to put down is also a joke about anti-gravity: if gravity pulls things down, perhaps anti-gravity does the opposite and makes them impossible to put down.