The Theory of the leisure class - By Thorstein Veblen
I can't recommend this highly enough, there are so many ideas and concepts that have shaped and sharpened my thinking on everything from the dynamics of class identity to human nature; if (like me) you dont have the time/energy/focus to read full books regularly, read a summary. The central idea of the book is that the primary class division is between those who make a living by exploiting others, and those who make a living by actually making stuff. Veblen wrote during the 'gilded age' (think the Rockefellers, the Vanderbilts, Carnegies etc). Personally he was an instinctive non-conformist, he didn't really care about politics, ideology or the working class (or that his boss was married to his mistress), but his critique and analysis of capitalism and class is unique and refreshing; I value it far above Marx's work, particularly because unlike Marx he looks primarily at what motivates the individual, how that influences class culture and how class culture influences that, where as Marx largely ignores the individual, using class as his starting point/unit of study/basic agent. Veblen's account of capitalism just feels more 'common sense', you don't need to immerse yourself in an ideological-pseudo-religion to 'get' it. Key ideas to look out for are 'emulation', 'conspicuous consumption' and 'the engineer's spirit'.
The Picture of Dorian Grey - Oscar Wilde*
I read Dorian Grey when I was 16 or 17, and it's proved a profound and lasting influence. It was basically my introduction to amorality, and I found the character Lord Henry to be particularly inspiring with his irreverent self-interested hedonism. When I first encountered egoism, it's ideas felt instinctively familiar to me, and I think a large part of that was this novel, or at least my selective reading of it.
Industrial Society and its Future (aka The Unabomber Manifesto) - Theodore Kaczynski
A thought provoking attack on civilisation and leftism, written by the Unabomber. I particularly like the idea of 'oversocialisation', it explains a lot of the the behaviour of the aggressively moderate liberal activists that pissed me off back when I was involved in activism.
Why Socialism? - Albert Einstein
I'm including this not because I agree with it, but because it's an interesting article, mainly because Albert Einstein wasn't an economist, an ideologue or a marxist. Because his expertise was in a completely different field his criticism of capitalism is pretty unique, and while it buys into the myth of progress to a certain degree, it's still worth a read.
*While Oscar Wilde is often identified as an anarchist of some description, I'm including him because his primary project/focus wasn't explicitly political.
Re: the Situationists, I've not read the actual texts very deeply but Raoul Vaneigem's 'The Revolution of Everyday Life' seems to be the most anarchism-oriented of the main Situationist texts, and is meant to be an easy and entertaining read.