Shelley uses a wide array of figurative, or non-literal, language in "Ozymandias," giving great depth to the sonnet. The way that the poet uses the word "read" is an idiomatic use of the word. Usually we think of printed words being read, but we also speak of reading the minds or emotions of others. By using the word "stamped" in the next line, the poet further suggests the metaphor of reading.
"The hand that mocked them" is an example of synecdoche, using a part to represent the whole, because it was the King, not just his hand, who mocked them. The word "mocked" is certainly an understatement; while the powerful King may have literally mocked his subjects, chances are his treatment of them included much worse, such as brutal punishments and executions. "The heart that fed" is both synecdoche and a metaphor: The King, not just his heart, "fed" upon his people like a predator devours its prey.
"King of kings" is hyperbole; the King ruled his own kingdom, but there were certainly many other kingdoms that he had no control over and that did not even know of him. The word "decay" suggests symbolism; although buildings and structures can be said to decay, we normally think of the word in relation to bacterial decomposition of living matter, so the "decay" of the statue represents the rotting of the King's corpse, which has gone the way of all flesh. The word "colossal" might be an allusion to the Rhodes Colossus, a gigantic mythical statue that fell in an earthquake. "Boundless" is hyperbole, for the sands are not infinite.
The poem also contains further symbolism as well as irony. The statue, constructed on order of the King to symbolize himself, now lies wrecked in the sands, symbolizing the King's own fall into obscurity. The irony lies in the fact that the gigantic pride of the King has resulted in a gigantic fall.