It was mostly about the money - as it usually is. BOTH sides were motivated by their financial interests primarily. The Northern states wanted high tariffs so they could raise prices and still gain market share and so they could continue to gorge on government subsidies. The South wanted low tariffs so they could export the maximum possible and pay the lowest prices possible for manufactured goods.
As for the Civil War, the Old South plantation system ran on slaves. The small ongoing exactions of the tariff system on the South were a minor issue compared to the immense capital value of slaves and their central role in the plantation economy.
Only 5.63% of the total free population in the South owned slaves. Of those, half of them owned 5 or fewer. The large plantation owners were a tiny minority of the population and contrary to the PC Revisionist propaganda, no they did not control everything. To most Southerners, a massive hike in the tariff would be economically devastating - as indeed it was during the Tariff of Abominations 30 years earlier. In any event, slavery was not threatened in the US.
As for original sources in the era, secessionists largely discussed tariffs as the projected revenue source for the new Confederate government. They did not want to do away with the tariff. They wanted to keep it and use it to finance the Confederacy, thereby preserving slavery and the immense profits of the plantation slave system.
The Confederate constitution allows a revenue tariff (Maximum 10%) but not a protective tariff. The CSA was going to have low tariffs. Slavery was going to end sooner rather than later and it was especially going to collapse quickly without the benefits and protections of the fugitive slave clause of the US Constitution. Just about everybody understood that.
In economic terms, slavery permitted the plantation owners to avoid paying wages, keeping for themselves the value of the slaves' labor. When the slave holders of the Old South defended slavery, complained about fugitive slaves, and provoked secession, they had potent economic motives for doing so.
That would only be true IF
1. Slavery were being threatened in the US which it was not
2. secession would preserve slavery when in reality it would have the exact opposite effect.
"But secession, Lincoln argued, would actually make it harder for the South to preserve slavery. If the Southern states tried to leave the Union, they would lose all their constitutional guarantees, and Northerners would no longer be obliged to return fugitive slaves to disloyal owners. In other words, the South was safer inside the Union than without, and to prove his point Lincoln confirmed his willingness to support a recently proposed thirteenth amendment to the Constitution, which would specifically prohibit the federal government from interfering with slavery in states where it already existed." (Klingaman, Abraham Lincoln and the Road to Emancipation, pp. 32-33)
Again, the South's articles of secession cited slavery as the reason for secession, not tariff rates. In a era of substantial property requirements to vote, slave owners had the upper hand politically. In effect, non-slave holders were mostly disenfranchised.
As established, the Confederate central government began with a tariff rate of 12.5 per cent, but trade and revenue soon collapsed due to the Union blockade. The Confederate national and state governments then tried to finance themselves and the war effort through debt and currency printing, which resulted in catastrophic inflation.