Here's a good summary of the history of the
Second Amendment by law professors
Adam Winkler and
Nelson Lund. This is represents the mainstream view from balanced perspectives, and it's a quick read. Unless you want to spend weeks reading the
Federalist Papers and derivative interpretations, I suggest starting there.
If you want to get a sense for modern 2nd Amendment jurisprudence, read District of Columbia v. Heller, 554 U.S. 570 (2008) (holding that 2nd Amendment guarantees an individually held, enforceable right to bear arms); McDonald v. Chicago, 561 U.S. 742 (2010) (incorporating 2nd Amendment against the states through 14th Amendment Due Process clause), and Caetano v. Massachusetts, 577 U.S. ___ (2016) (holding, per curium, that the 2nd Amendment includes "arms" not in existence at time of nation's founding). Make sure to read the dissents and concurrences, because they'll give you a sense of where the genuine legal disputes are. If you want a sense of pre-Heller 2nd Amendment jurisprudence, read United States v. Cruikshank, 92 U.S. 542 (1875), Presser v. Illinois, 116 U.S. 252 (1886), and United States v. Miller, 307 U.S. 174 (1939). The latter three cases are good for historical perspective, but be advised that the Supreme Court has limited and / or distinguished them to the point that they are effectively overruled (at least so far as gun control advocates are concerned). The current composition of the SCOTUS is not terribly enamored of them.
The law among the circuits and states varies, as Lord Coke can tell you. In fact, I believe that in the next few years, the Supreme Court will issue a landmark ruling expounding upon the pronouncements in Caetano (remember, that case was per curiam, and was relatively short), and clarifying the limits of the state's ability to place restrictions on gun ownership. In particular, the recent case of
Young v. Hawaii, 896 F. 3d 1044 (9th Cir. 2018) arguably evinces a circuit split regarding so-called "open carry" laws. The case is pending en banc review in the Ninth Circuit, and may end up in the SCOTUS.