East of Eden is the story of two families in the Salinas Valley, the Hamiltons and the Trasks, and their seemingly inevitable reenactment of the biblical story of Cain and Abel. Steinbeck is perhaps best known for the ultra-realism of The Grapes of Wrath, which makes East of Eden, with its grand themes and supernatural allusions, a tough read for some fans (Just check the Amazon reviews for proof). Still, although I loved Grapes and consider that it is probably an objectively better book, East of Eden's mythical scope and archtypical characters drew me in even more.
The first half of the book tells the story of Adam and Charles Trask, and the second tells the story of Cal and Aron, their sons. There's a woman of course, in the form of Cathy Ames, one of the most vile characters (or creatures, as Steinbeck refers to her) in all of literature. Still, even in her, Steinbeck manages to inject the tiniest elements of humanity, although it's just enough to make her cruelty seem even more loathsome.
The theme of the story hinges on a difficult-to-translate word in Gensis 4, where the story of Cain and Abel is found in the Bible, a word that is determined to mean “thou mayest,” and the entire book is about choices. In spite of their circumstances, everyone in East of Eden decides their own fate by the choices they make, whether for good or for evil. It's a point examined again and again, sometimes subtly, as in the main storyline, and sometimes a bit ham-fistedly, as in the monologues by Adam's manservant, the somewhat stereotypical Chinaman Lee. The occasional didacticism is the main downfall of East of Eden, although to me, it just added to the mythical nature of the whole story. And, that's all I have to say about that.
Oh, one more thing. If you've seen the movie, you should be aware that it only covers roughly a third of the book. The part it leaves out would never make it into a PG rated film.