By "language of a book" you mean, I think, the terminology of literature, the language we use to talk about writing. Diction concerns the choice of words to be used. Here is the closest I have come for a definition of literary device:
I hope that helps.a particular word pattern, figure of speech, combination of word sounds, etc., used in a literary work to evoke a desired effect or arouse a desired reaction in the reader: rhetorical devices.
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