Property is Theft!, which ought to be available any day now. There are other sale prices out there, but none that give you a good deal and also put the revenue in the hands of the radical business responsible for making this important material available.
The traumas of the book industry are in the news again, and the pundits are really just scratching the surface of what went wrong and what could go even more wrong if the industry continues to centralize. Whether they like it or not, consumers are going to be choosing sides in a series of complex battles to determine the future shape of the industry. If you care about the publication of books like this one, it's a bit of a no-brainer that the clearest signal and the most efficient aid you can deliver is to purchase directly from AK (or from your local bookstore or infoshop.)