essential

Generally, I don't like short books, but two that I do like are Harold Bloom's The Anxiety of Influence and A Map of Misreading. 

The Anxiety of Influence is the core of his theory, and then in A Map of Misreading, he begins to apply the theory to reading. 

After those 2 he continues to expand on his theory and his books get longer, and some of them are great, but they aren't essential. You can get a really good grasp of Bloom's theory by reading and rereading those first two.