"He claims for free negroes the right of suffrage, and an equal voice in the Government-- in a word, all the rights of citizenship, although the Federal Constitution, as construed by the highest judicial tribunal in the world, does not recognize Africans imported into this country as slaves, or their descendants, whether free or slaves, as citizens. These were the issues presented in the last Presidential canvass, and upon these the American people passed at the ballot-box...Upon the principles then announced by Mr. Lincoln and his leading friends, we are bound to expect his administration to be conducted. Hence it is, that in high places, among the Republican party, the election of Mr. Lincoln is hailed, not simply as a change of Administration, but as the inauguration of new principles, and a new theory of Government, and even as the downfall of slavery. Therefore it is that the election of Mr. Lincoln cannot be regarded otherwise than a solemn declaration, on the part of a great majority of the Northern people, of hostility to the South, her property and her institutions-- nothing less than an open declaration of war-- for the triumph of this new theory of Government destroys the property of the South, lays waste her fields, and inaugurates all the horrors of a San Domingo servile insurrection, consigning her citizens to assassinations, and her wives and daughters to pollution and violation, to gratify the lust of half-civilized Africans. Especially is this true in the cotton-growing States, where, in many localities, the slave outnumbers the white population ten to one. " - Letter of S.F. Hale, Commissioner of Alabama to the State of Kentucky, to Gov. Magoffin of Kentucky
You are correct to find an extensive source of information on secession there, simply be mindful that it is a representation of primarily the northern side of the argument. With all due respect to the site's authors, that site is more or less designed to argue for the identification of slavery as the issue, and I believe the site even admits so.
While it is an excellent source of documentation for various pro-slavery arguments, to imply those arguments to represent the entire spectrum of the debate is misleading and incorrect.
Notice that you will not find a number of documents there that are often prominently placed among the secession papers. The Cherokee declaration of causes, which barely even mentions slavery but instead concentrates heavily on northern abuses of the constitution, is not there. Nor is the Arizona secession ordinance, which cites the failure of the yankees to equip their frontier as their cause. Nor are any of the other Indian treaties of alignment with the confederacy as far as I can tell. A couple pro-slavery newspaper editorials are listed, but not the many tariff and pro-secession editorials from the same time. Aside from the Davis farewell and a select few of the big southern speeches, very little is there to represent the southern statements during the winter session of congress in 1860-61. Comparatively, Lincoln's major speeches on slavery are all there (but not the one where he says that tariffs are his top priority of the next congressional session). Nor are the winter session's economic speeches to the Senate and House.
There is no mention of the speech where Senator Wigfall, one of the leading secessionists, took up the secession cause in strictly economic terms. Though few know of the speech itself today, it left at least one linguistic on history which I am sure you can identify. Every school child in America has heard its opening line, "I say that cotton is king, and that he waves his scepter not only over these thirty-three States, but over the island of Great Britain and over continental Europe." But more important than historical catch phrases, the speech directly asked the economic question and on the grounds of the economic question, not the slavery one, concluded "I would save this Union if I could; but it is my deliberate impression that it cannot now be done."
The site similarly neglects the major pro-conciliation speeches made by northerners. On the eve of the first wave of secession, Charles Francis Adams pleaded the cause of conciliation before Congress and essentially layed the blame for the secessionist impulse on an anti-compromise faction of northern radicals. The speech was very famous in its own day and circulated heavily in print outside of congress but has since been rejected.
It is things such as this you should keep in mind when attempting to uncover the spectrum of political opinions on secession. References to slavery are there and find plentiful circulation on the web and in the history books. This should come as no surprise as they give support to what has become the official line of the yankee account of the war as well as the position of political correctness. But as any thorough examination of history will reveal, the record is frequented by other causes than slavery that have since been neglected due to their incompatability with the favored line given by historical frauds such as McPherson.