The idea that the Civil War was not completely about slavery is a bit of a bald man fallacy. The other issues were mostly ancillary to the slavery debate.
Four of the states published formal Declarations of Secession. Each mentions slavery first and foremost:
Virginia
The people of Virginia, in their ratification of the Constitution of the United States of America, adopted by them in Convention on the twenty-fifth day of June, in the year of our Lord one thousand seven hundred and eighty-eight, having declared that the powers granted under the said Constitution were derived from the people of the United States, and might be resumed whensoever the same should be perverted to their injury and oppression; and the Federal Government, having perverted said powers, not only to the injury of the people of Virginia, but to the oppression of the Southern Slaveholding States.
Texas
Texas abandoned her separate national existence and consented to become one of the Confederated Union to promote her welfare, insure domestic tranquility and secure more substantially the blessings of peace and liberty to her people. She was received into the confederacy with her own constitution, under the guarantee of the federal constitution and the compact of annexation, that she should enjoy these blessings. She was received as a commonwealth holding, maintaining and protecting the institution known as negro slavery-- the servitude of the African to the white race within her limits-- a relation that had existed from the first settlement of her wilderness by the white race, and which her people intended should exist in all future time. Her institutions and geographical position established the strongest ties between her and other slave-holding States of the confederacy. Those ties have been strengthened by association. But what has been the course of the government of the United States, and of the people and authorities of the non-slave-holding States, since our connection with them?
South Carolina
The people of the State of South Carolina, in Convention assembled, on the 26th day of April, A.D., 1852, declared that the frequent violations of the Constitution of the United States, by the Federal Government, and its encroachments upon the reserved rights of the States, fully justified this State in then withdrawing from the Federal Union; but in deference to the opinions and wishes of the other slaveholding States, she forbore at that time to exercise this right. Since that time, these encroachments have continued to increase, and further forbearance ceases to be a virtue.
The SC declaration then goes on a long walk about the revolution and their right to secede before getting to their reason why: the northern states are disregarding fugitive slave laws.
Mississippi (particularly ugly)
In the momentous step which our State has taken of dissolving its connection with the government of which we so long formed a part, it is but just that we should declare the prominent reasons which have induced our course.
Our position is thoroughly identified with the institution of slavery-- the greatest material interest of the world. Its labor supplies the product which constitutes by far the largest and most important portions of commerce of the earth. These products are peculiar to the climate verging on the tropical regions, and by an imperious law of nature, none but the black race can bear exposure to the tropical sun. These products have become necessities of the world, and a blow at slavery is a blow at commerce and civilization. That blow has been long aimed at the institution, and was at the point of reaching its consummation. There was no choice left us but submission to the mandates of abolition, or a dissolution of the Union, whose principles had been subverted to work out our ruin. That we do not overstate the dangers to our institution, a reference to a few facts will sufficiently prove.
http://avalon.law.yale.edu/subject_menus/csapage.asp