The classics, the most famous books of the last few thousand years, are a good bet. They've appealed to many generations. You have to read them to find out which ones speak to you.<p>For me, I'm constantly rereading the essays and non-fiction/philosophical writings of Hazlitt, Emerson, Chesterton, Stevenson, Santayana, William James, Collingwood. For 20+ years so far. Others, like Nietzsche, Wilde, Hilary Putnam, Dewey, I don't read so constantly but I've read some/most of their books over and over again. Wilde's <i>Intentions</i>, Putnam's <i>Realism with a Human Face</i>, Dewey's <i>Art as Experience</i>, for example, and almost everything of Nietzsche's, e.g. <i>Human, All Too Human</i> or <i>Daybreak</i>, I find endlessly rereadable. Chesterton, Nietzsche and Wilde are very funny too.