The Anarchist Library is a great archive of free books and essays on the topic of Anarchism. Some newer books won't be available there due to copyright (thanks Capitalism! /s).
*Dear mod: I don't think there's any copyright concerns with the linked texts, but if there is I'll remove the direct links. I only added them because the search function on the archiveis a bit borked, so it can be hard to find what you're looking for.