Errico Malatesta is a rather underrated anarchist theorist and I thoroughly enjoy his work. I know you probably don't want long books, so I'll recommend a couple short ones that are intentionally brief. First, his text simply called "Anarchy" is probably the best primer there is, despite the bread book often being recommended as the first read. Malatesta also has a very interesting text called "At The Café" that works similarly as a primer, but done with actual characters, almost like a Socratic dialogue. I read this one as part of a socialist book club and it was a lot of fun.