Alexander H. Stephens' book, A Constitutional View of the Late War Between the States, has been posted online at constitution.org. In a series of conversations ("colliloquies") originally published in 1868, the Confederate Vice President explains the relationship between the Federal Government and the States from the Constitution itself, the writings of the Founding Fathers, and the ratifying debates. He also discusses the views of Daniel Webster and John C. Calhoun, and relates it to the Confederate cause during the War Between the States.
The .html file is not terribly easy to read, being a direct scan from the original, which requires some adjustment to reading the footnotes as they appeared at the bottom of each page in the original. It also contains a number of helpful appendices, including the Confederate Constitution. On the other hand, the book is difficult to find, and is expensive to purchase (I bought mine about ten years ago for $80.00).
It is excellent reading for anyone who wishes to understand the Constitutional and legal bases for State sovereignty.