I ran to the library to check up on the comment about secession being illegal as well as the federal government's attempt to "pursue them" in Shelby Foote's "Fort Sumter to Perryville." I'm not sure if this was the part you were talking about (since you have not yet given details), but I find on page 18 (emphasis added):

"Change was predominant not only in Springfield; the Union appeared to be coming apart at the seams. Louisiana and Texas had brought the total of seceded states to seven. Banks and business firms were folding; the stock market declined and declined. JAMES BUCHANAN, badly confused, was doing nothing in these last weeks of office. Having stated in HIS December message to Congress that while a state had no lawful right to secede, neither had the federal government any right to prevent it, privately he was saying that he was the last President of the United States."
Shelby Foote, "The Civil War: A Narrative. Fort Sumter to Perryville," p. 18.

This seems to be exactly what you were referring to. Foote, in no uncertain terms, explains that this was Buchanan's position...not Lincoln's. Is there somewhere else in the book that Foote claims Lincoln disputed federal authority to resist secession?