I am interested in the Legal System of the USA. I am very impressed. Which is the best textbook for it?

Can I suggest you start with this book https://www.amazon.com/Violence-Social-Orders-Conceptual-Interpreting/dp/0521761735#reader_0521761735 It will give you a superb grasp of the concepts that underlie Common Law. It is quite scholarly so not a quick read but it is not indigestible.

Essentially it is economic history presented through a Libertarian lens. As the writers of the US constitution were Libertarians, this will help you understand where they were coming from.

"The USA" is a big place and "the law" is a huge domain, with a lot of variation at the state level. Depends what branch of law you're looking at whether laws are primarily federal, state or local. This sounds like you haven't even started looking at things, so any basic book on the US constitution to start, and then look at specific domains of law as you see fit.

The US Constitution is the first place to start .

The best place to start would be with the federal income tax code for individuals. It is easier than the one for corporations. google should be able to lead you to it.