For hard copy English learning books that can help you learn English, you can go to your local library and borrow them for free. In some libraries, however, you'll have to pay a membership and then very small fees to get the books.
As I said previously, learning English with books is sooo 2000. The modern way to learn this language is through online courses. Google "BBC English" without the quotes to see what I mean. BBC has one of the most extensive digital libraries to help you learn this language without having to pay nothing.
You can also use the classic way: searching the web. But I can't guarantee you how good the books you'll find are for mastering the English language. Most of them are very bad. The reason for this is because they throw you bunch of "tips and tricks" and don't give you a FOUNDATION which is the most important thing.
So I recommend you avoid this and start with some really good and credible resources as BBC English.
At the end, I want to mention that if you think that a book only is going to help you learn English, you're dead wrong. Books develop your reading skills. But you have 3 more skills to be aware of when learning any language: speaking, listening and writing.
There are some great software applications that incorporate all of these styles like Rosetta Stone (which is not free but great if you're starting to learn English.)