Here are the remaining texts. The tally is now 28, I forgot to include several that are important, there's also several 'afterthoughts'.

19. Psychopathology of everyday life—Sigmund Freud. This text may seem a strange inclusion but some of Freud's writings are considered philosophical works. Here, I refer you only to Chapter 1 titled The forgetting of Proper Names. It's only a dozen or so pages so it's a quick read. Whether Freud's analyses is correct or wrong, or that modern science has superseded his work is immaterial here. It's importance to philosophers is that it vividly conveys how a great mind broaches a subject and tackles the analysis and comes to conclusions (it's a rare and insightful opportunity to see a philosopher's mind at work). If you are keen also read Chapter II about the Forgetting of Foreign Words.

20, 21, 22. The Logic of Scientific Discovery | The Open Society and Its Enemies, Vol-I Plato | The Open Society and Its Enemies, Vol-II Hegel & Marx — Karl Popper These books represent only a fraction of Popper's output, albeit influential ones. Popper was a highly influential 20th C. philosopher and his fields are many and diverse—from biology, to the scientific method, to the structure of society, to exploring the subject of knowledge—critical rationalism, etc. If you're already studying HPS then it's likely The Logic of Scientific Discovery is one of your curriculum texts. It's here that Popper argues the notion that made him so widely known in the scientific world—that is, scientific methodology should be based on falsifiability, as experiments cannot prove a theory no matter how many times they're repeated but reproducible experiments can refute or disprove a theory.

The Open Society and Its Enemies is a two volume set. For the moment limit yourself to Vol. I (Vol. II is mentioned to avoid confusion). Like the Austrian School of libertarian economists (Ludwig von Mises, Friedrich Hayek), Popper was heavily influenced by his experience of totalitarianism—communist and fascist regimes before and during WWII, thus his antiauthoritarian views. He argues in favor of the free market, free trade, egalitarianism and liberalism and libertarianism thought. In Volume I he aims criticism at the Platonic concept of the benevolent philosopher king who rules through wisdom and experience. Popper views such rule as totalitarianism even if the dictator/despot is benevolent and wise. He argues that a tolerant society must be tolerant of intolerance—if not, then it too is intolerant. That said, matters aren't quite that simple, he then leads us to the paradox of tolerance, which states that if a society sets no limit on its tolerance then eventually the intolerant will destroy it.

Given current politics where both sides of the political divide are becoming more belligerent and intolerant of each other by the moment, Popper's views in The Open Society and Its Enemies are both strongly argued and very prescient in our troubled times.

(Note: in my comments on The Republic, my suggestion was that one should only confine oneself to Book I. This was for good reason; as later in Book IV Plato develops the notions of society's guardians and philosopher king and this isn't relevant to the matter of formal argument. What's notable here about Book IV popping up here is that a common characteristic of philosophical thought and argument is that new works builds heavily upon past thought. Thus, understanding older philosophical arguments is so important; it's why we still consider Greek philosophy a crucial starting point. Moreover, it illustrates the why one should progress through a formally structured curriculum. As with any study, to stay on course and to progress quickly certain topics are essential prerequisites.

23. Being and Nothingness—Jean-Paul Sartre* This text is one of the pillars of 20th C. existentialism, it's considered a true tour de force in the subject. It deals with one's role in the world—of being an existential human being and everything that implies. I haven't time to delve further here as I'd need to illustrate with longwinded examples. However, I'd suggest one should learn the essential basics of existentialism given that its philosophy is so ingrained in 20th Century thinking. The text's likely too long for an introductory course, so one could consult a précised synopsis (reckon it'd still be considered a 'legitimate' learning exercise (I can hear my once lecturers wincing and grinding teeth at the thought!).

24. No Exit - (Fr. Huis clos)—Jean-Paul Sartre The French title translates into English as 'In Camera' although it's better known as No Exit in the Anglophone world. No Exit is a famed but short [hence quickly read] play by Sartre with an existential undercurrent. It's an overstatement to say the play has a plot, rather it describes a situational existential existence more akin to, say, an extreme version of Samuel Beckett's Waiting for Godot. The play's importance to philosophy is that it examines the human condition—of existence—from the most extreme existential perspective possible. The play depicts a state of hell for those involved. Imagine yourself trapped forever in a room with people you hate or with whom you disagree but with never any possibility of escape. It's an extremely thought-provoking work.

25. Formal/Symbolic Logic Texts I can't provide a list and do justice to it. There are too many and I'm not conversant with the latest ones. However, you must study Formal Logic as, at minimum, it's necessary rigor for one to develop logical thinking (there are also many other reasons). It teaches one to accurately comprehend thus determine the meaning of what is written within texts. Often one's comprehension of written text is superficial and imprecise thus one's perceived or interpreted understanding of it is either a poor chimera of the original or perhaps altogether wrong.

Formal logic provides one with tools—symbolic terminology, methodologies and mathematical logic to analyse statements and propositions then reduce them into logical arguments. Sentences are broken down and atomized into subjects, predicates etc. using true/false truth tables along with other formal procedures. Advanced techniques lead to a detailed Semantic theory of truth as formulated by a long line of logicians from George Boole, to Russell, to Wittgenstein through to Alfred Tarski and others. Formal/symbolic logic is a major and necessary extension to Boolean logic, here's a glimpse: https://en.wikipedia.org/wiki/List_of_logic_symbols

(Note: Logic, in its various forms, is a sub branch of mathematical philosophy of which of itself is an enormously broad subject from before the Ancient Greeks onward to modern mathematical thought of Russell, Wittgenstein, Kurt Gödel (his incompleteness theorem) and numerous others. It's likely this topic is covered in your HPS curriculum, thus it's a redundant exercise for me to do so. In addition, I'm not a mathematician by trade, hence if I offered further advice I'd be doing the subject an injustice.

26. Stanford Encyclopedia of Philosophy: https://plato.stanford.edu/index.html Stanford Encyclopedia of Philosophy is, I'd reckon, the most comprehensive philosophy resource on the web. It covers almost every phil. topic imaginable from ancient Greece to HPS and everything else. Its breath of knowledge intimidates me, in fact almost everyone. Don't let that stop you using it as a primary reference source.

Here are some important mainstream topics not covered in my list that you'll eventually encounter (IMHO, they aren't necessary HPS prerequisites): Geek Epicureanism/epicurean philosophy, Epictetus and other Greek philosophers, Thomas Aquinas, Friedrich Nietzsche, Mary Wollstonecraft, Baruch Spinoza, Georg W.F. Hegel, Karl Marx and 20th Century philosophers including, Martin Heidegger, Jacques Derrida, Michel Foucault, John Rawls, John Ralston Saul and others. (Surely, I've missed some key names but there's plenty here to start with.)

An afterthought, perhaps I should have included these two texts:

27. Tractatus Logico-Philosophicus (aka TLP & Treatise on Logic and Philosophy) — Ludwig Wittgenstein In TLP, Wittgenstein concerns himself with the 'relationships' between logistics of language and reality, how this 'understanding' defines the scope or limits of science. This book has had considerable influence on 20th Century philosophers (Russell, G.E Moore and many others). Note: understanding language semantics and how it plays a role in philosophical thinking, perception and logic is a major aspect of 20th C. philosophy.

28. Language, Truth, and Logic — A.J Ayer I initially omitted this book, as later Ayer was critical of it stating that it contains mistakes of his youth. On second thoughts, I've included it, as the way Ayer explains language, the logic of language structures—propositions, statements (such as tautologies being necessarily true and thus don't need verification), etc. had a defining influence on me (in that they sunk into my grey matter first time). However, I acknowledge this is not sound logic (my only defense is that it was an easy text to digest).