In East of Eden, who is telling the story?

I'm currently reading East of Eden. I'm just starting Part 3 (chapter 23). I seem to have followed everything up to this point and I think I understand who all the characters are and what their relationship to each other is. But suddenly Part 3 is written in the first person. There are many references to "I" and "me" but I don't know who is talking (writing). Who is "I"? This is likely not an oversight on Steinbeck's part, but why is he suddenly writing in the first person? Did I miss something that explains what's going on?