You came upon me carving some kind of little figure out of wood and you said, "Why don't you make something for me?"
I asked you what you wanted, and you said, "A box."
"To put things in.
"Whatever you have," you said.
Well, here's your box. Nearly everything I have is in it, and it is not full. Pain and excitement are in it, and feeling good or bad and evil thoughts and good thoughts—the pleasure of design and some despair and the indescribable joy of creation.
And on top of these are all the gratitude and love I have for you.
And still the box is not full.
First things first: who is Pascal Covici? Answer: he was Steinbeck's editor. This epigraph is less of a thematic intro and more of a thank you to the guy who helped make the novel possible.
But it's also interesting that Steinbeck focuses on the idea of his book as a box into which he puts things, rather than something that he just creates and is done with.
In this case, it's really the things that are in the box that matter more than the box itself. But the box is filled with some pretty big things: pain and excitement, evil thoughts and good thoughts—"and still the box is not full." The point is, East of Eden is vast and contains a lot of huge concepts, not to mention a healthy dose of blood, sweat, and tears. We're talking enormous ideas, as in good and evil locked in a deadly battle for all of humankind, as in Epic-with-a-capital-E.
All that in a half-empty box.