First, you should know that this book, though the basis of the Iron Giant animated film, differs from it significantly. The early parts of the book, in which Hogarth meets the giant, are similar, but the ultimate conflict is totally different. The almost overt christian symbolism of the movie (in which the giant sacrifices himself for our sins and is reborn), is much more subtle in the book and perhaps not there at all. The earth is threatened by a monster from space and the giant subjects himself to great suffering in order to trick the creature into self-immolation.
Sound like kiddie stuff? Oddly, my four-year-old son loves this book and I have read it to him many times. Perhaps he can relate to the situation of Hogarth and can imagine himself as defender and champion of this massive, mysterious creature. I think he also likes the language, which is repetitive without ever being cloying. Hughes has no fear of long compound sentences or polysyllabic latinate words.
The lesson of the book is similar to that of the movie, about assuming the best of others, and taking risks to defend those you care for, but the book is more sophisticated. The art in this edition is lovely, and fits the tone of the book much better than stills from the movie possibly could.