The first of his books to use aphorisms - though certainly not the last - Human, All Too Human breaks away from Friedrich Nietzsche's earlier book, The Birth of Tragedy, in both style and philosophy. Here we have this great writer's transition into the middle period of his career, with a strong emphasis on the metaphysical; in Nietzsche's opinion, in reference to the title of this volume, only through understanding can we overcome what makes us "all too human." Though we recognize the brilliance of this thought today, at the time of publication in 1908 (in English), he was, alas, poorly received.
Thus Spoke Zarathustra The Gay Science Human All Too Human
For some reason i don't often see this book mentioned. Perhaps i am biased (this was my introduction to Nietzsche) but i have always found this to be more nuanced, incisive, insightful and less polemical than his other works. Also the fact that it is basically a set of aphorisms (of varying para lengths) on very many topics means that you can read it from any page for short periods of time and still find something of value.
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 Intentions, Putnam's Realism with a Human Face, Dewey's Art as Experience, for example, and almost everything of Nietzsche's, e.g. Human, All Too Human or Daybreak, I find endlessly rereadable. Chesterton, Nietzsche and Wilde are very funny too.
You kind of need to get into his train of thought to appreciate what he's getting at.
He was a stark individualist that put into question a lot of the building blocks of liberal democracy, such as the notion that all men are equal. I don't agree with all of what he puts forth but he is without a doubt one of the most dynamic and influential thinkers of the 19th century and still relevant to read even today.
If you're interested in reading him, start with Twilight of the Idols and Beyond Good and Evil. If you like those two then you can delve into some of his other work such as the Antichrist, On the Geneaology of Morals, Human, All Too Human, and Thus Spoke Zarathustra
If you don't want to just read Kenny's ANHWP, and I were to provide a reading list:
* Plato: Meno, Gorgias, Parmenides, Charmides, Apology. If you enjoy it, then try reading The Republic.
* Aurelius' Meditations. Democritus and Epicurus are worth reading about too, to summarise the Greeks. And Zeno's paradoxes.
* meaningness.com as an entry-point to Buddhist philosophy in general.
* Logicomix as an introduction to Russell & his project in Principia Mathematica.
* Mill's Utilitarianism, Hannah Arendt 'The Human Condition', and Henry George 'Progress & Poverty' for politics, history and economy, throw in a bit of Rawls. Schiller's 'The Robbers' is also very much worth reading.
* SEP articles for the classic writers: Augustine, Aquinas, Hume, Kant; then Kripke, Tyler Burge, and Searle present some interesting problems.
Other mentions off the top of my head: Dreyfus, Chalmers, Parfitt, Singer.
There are some great YouTube channels out there that teach to the intelligent adult. Gregory B Sadler, Mark Thorsby, Daniel Bonevac, Carneades.org are the ones I remember being useful. Some good series' are out there: Human, All Too Human; Alan Watts' TV series; The Reith Lectures...
I've missed a ton, of course, this is just stuff I remember enjoying or gaining something from. I hope this list isn't too overwhelming. I'd start with the Greeks, then flick around some YouTube channels, then meaningness, then off into whatever takes your fancy. I don't really fancy digging through MOOCs so sorry I can't recommend any in particular.
My personal favourite philosophers/philosophical writers: Nietzsche, Wittgenstein, Darwin, Hegel, Camus, CS Pierce, Feyerabend, Epicurus.