Many of these books are open books or out of print books. e.g. O'Reilly has its open books at http://www.oreilly.com/openbook/. So these are supposed to be legal. And if by mistake you download some pyrated copy then nobody is gonna hang you till death so dont worry much. Look I am not encouraging pyracy, even I am against pyracy and cracking, and many pyrated copies have malwares attached, but now a days pyrated copies look so much real that they are hard to distinguish. So dont think much about this and try to get more and more resources to learn. Coz learning is very much ethical purpose.