In the text, "Noah remarked that the new book on anti-gravity was impossible to put down," the figure of speech used is a pun. This is because the phrase "impossible to put down" has a double meaning: it suggests that the book is so engaging that one does not want to stop reading it (a figurative interpretation) while also referencing the topic of anti-gravity (a literal interpretation).
Additionally, it could be argued that there is an element of hyperbole involved, as saying a book is "impossible to put down" is an exaggeration for effect.
So the two figures of speech present are pun and possibly hyperbole.