'Para' is 'for' in this sentence.
...para aprender inglés = for learning English
This tells you the reason why you are doing something.
When you're talking about a book, or a course, or a lecture, or an article, for example, the preposition 'on' means 'on the subject of'. For example, if you are interested in archaeology, you'd say
'I want to borrow a book on archaeology.'
I hope that helps.