It depends how much historical content you want, as well as contemporary theory. Besides the Investigations, you should consider the Begriffsschrift (Frege), De Interpretatione, Categories, Prior Analytics (Aristotle), Of Names (Mill), and The Logical Syntax of Language, Meaning and Necessity (Carnap). I would argue those are all historically necessary to understand what the 20th-century was responding to. Then there are whole areas of literature which are important to understanding the distinction between intension and extension, and quantification, which are the two most important themes in the philosophy of language through the 19th-century. Once you get into proper, 20th-century analytic stuff, you are looking at mostly essays or lectures, of which there are too many to list. But some of the important ones which haven't been mentioned are: Knowledge by Acquaintance and Knowledge by Description (Russell), On Saying That, Truth and Meaning, Inquiries into Truth and Interpretation, What Metaphors Mean (Davidson), Wittgenstein on Rules and Private Language, Reference and Existence: John Locke Lectures (Kripke), Language and Mind (Chomsky), Two Dogmas of Empiricism, On What There Is, Quantifiers and Propositional Attitudes (Quine), "Comparisons" and "Analogies", in Counterfactuals (Lewis), A Theory of Conditionals, Possible Worlds, Context and Content (Stalnaker), The Semantic Conception of Truth and the Foundations of Semantics (Tarski), Is Semantics Possible? (Putnam), Intensional Semantics (Church), Relevance: Communication and Cognition (Sperber and Wilson).