I aced a basic logic class in college that covered both sentential and predicate logic. I am interested in furthering my skills in symbolic logic, but I don't know how. My school doesn't offer any upper-level logic courses. I'm thinking I would like to buy a simple textbook for a more in-depth study of the more advanced concepts (I've heard the term "modal logic" thrown around, but I don't know what that is). Can you suggest a good text or author I should investigate?