There is no need for you to buy law books in order to get ready for law school. They will teach you all you need to know.
However, what is important in order to get into your LLM program and succeed in law school is a pretty good knowledge of the language.
Instead of using your time to study US law (you will study the same stuff during your program), I suggest you take English classes. Once you are able to fluently read, write, and speak in English, you'll find out that learning US law is not that difficult even for a civil law student.