Here are few more sites that provide email tracking services

http://www.didtheyreadit.com/

http://www.epagecollection.com/tools/emailtracker/index.html

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...

http://www.freeprogrammingebooks.com/

http://www.epagecollection.com/

http://www.docnmail.com/learn/

http://www.freebyte.com/programming/cpp/
...