
Whatever you decide about his beliefs, Nagel is a wonderful writer. Pick up a copy of Mortal Questions if you get a chance.
Philosophical Investigations is beyond my ability to follow as is Heidegger (I think it's a common complain against continental philosophy in general). Kripke's Naming and Necessity is something I can understand, but I don't really feel the 'oomph' to the insight that there can be a posteriori necessary truths and apriori contingent truths. Obviously it's a deeply important fact, and an astonishing breakthrough. But it doesn't hit me in the same way as Nagel's Absurd, which attempts to break down and dismiss existential dread. I don't think he succeeds, exactly, but it feels important and deeply insightful.
Some from the canon:
Plato: Defence of Socrates, Euthyphro, Crito (translated by David Gallop)The Modes of Scepticism by Julia Annas and Jonathan Barnes (a beautiful collection of texts from Pyrrhonian scepticism and analysis of their arguments; the best way I know to meet ancient scepticism)
Descartes: Meditations on First Philosophy (The Cambridge University Press edition is very good; it's probably worth buying their Selected Philosophical Writings for a good basic Descartes text)
David Hume: An Enquiry concerning Human Understanding and An Enquiry concerning the Principles of Morals (I recommend the Oxford Philosophical Texts editions)
Wittgenstein: Philosophical Investigations (terribly, terribly difficult, but very rewarding)
A few contemporary things:
Gilbert Ryle: The Concept of MindWillard Van Orman Quine: From a Logical Point of View
J. L. Mackie: Ethics: Inventing Right and Wrong
Thomas Nagel: Mortal Questions (short essays, well-written, very accessible)
Bernard Williams: Ethics and the Limits of Philosophy
Michael Smith: The Moral Problem
Specialist, technical, odd:
The Frege Reader by Michael Beaney (for modern logic and the beginnings of contemporary philosophy in the analytic style)J. L. Mackie: The Cement of the Universe: A Study of Causation
William Kingdon Clifford: The Ethics of Belief (he argues, oddly but powerfully, that it's immoral to believe things on insufficient evidence or irrationally - an offense to morality, not just reason)
Dennett denies Nagel's claim that the bat's consciousness is inaccessible, contending that any "interesting or theoretically important" features of a bat's consciousness would be amenable to third-person observation. For instance, it is clear that bats cannot detect objects more than a few meters away because echolocation has a limited range. He holds that any similar aspects of its experiences could be gleaned by further scientific experiments.
-- https://en.wikipedia.org/wiki/What_Is_it_Like_to_Be_a_Bat%3FHeterophenomenology ("phenomenology of another, not oneself") is a term coined by Daniel Dennett to describe an explicitly third-person, scientific approach to the study of consciousness and other mental phenomena. It consists of applying the scientific method with an anthropological bent, combining the subject's self-reports with all other available evidence to determine their mental state. The goal is to discover how the subject sees the world him- or herself, without taking the accuracy of the subject's view for granted. -- https://en.wikipedia.org/wiki/Heterophenomenology