I said not necessarily a pirate. If you bought the book knowing that you were purchasing a pirated book then you are a pirate. If you unknowingly bought the book from a pirate site, but paid so much less that you should have known the site was pirating, then you are also a pirate. If you bought from a pirate site, but paid what is considered a fair price you have a good chance at getting off if charged as a pirate. This is in the US.
Another example using stolen merchandise. and pirating is stealing.
If you knowingly buy stolen merchandise you are also criminally liable. If you bought the merchandise not knowing that it was stolen and paid so low a price that you should have known it was stolen, you are criminally liable. if you paid, what the courts determined was a fair market price, you are not criminally liable.