As you begin your research in American Indian Law, it is important to distinguish between Tribal Law and Federal Indian Law. Tribal Law is the laws of the individual Indian Nations. Federal Indian Law is the law of the United States as it has developed in the U.S. Constitution, by Congress, and in the Courts.
American Indian Nations are referred to as the "third sovereign" in the United States, despite being the original sovereign, historically. The legal relationship between the indigenous nations of the northern hemisphere and what would become The United States of America began with a series of Treaties starting in 1722 and the Congress of the United States ended treating making with Indian Nations in 1871.