East of Eden is his last book (I think). Its also one of his most complex, at least, in scope and characters and content. I think right after this, you would rather enjoy Tortilla Flat. Very different, and very humorous. Also, its not too big. Of Mice and Men, Cannery Row and Tortilla Flat are three great short Steinbeck novels.
Nope . . . The Winter of Our Discontent is his last novel (and the best I might add), especially for a male in the middle of his life.
Cannery Row followed by the sequel Sweet Thursday