I didn't find Spooner in a Civil War almanac or in my biographical section of my 1967 dictionary. There were some other very important abolitionists linked to Stowe and Garrison:
Theodore Weld was the eminence grise of the movement. Less well known because he wrote under pseudonyms and usually spoke in smaller venues, it was he who introduced the Beechers to abolitionism. Originally a seminarian interested in temperance, he came to the abolition issue in 1830, and introduced the Beechers, who were the children of a seminary president in Cincinnati whom Weld was hired to assist in 1831/2. He was a half-generation older than they (they were in their early 20's, she was still single), and he succeeded in getting himself dismissed over his "excessive" interest in the subject of abolition -- and took half the seminary's student body with him, including a young Edwin Stanton, who went on to read law instead -- an early warning of Stanton's character in office. Weld worked for the New York Emancipator for a while, and later on at another periodical serialized Uncle Tom's Cabin for Stowe in 1851. She also owed him a literary debt, which she acknowledged publicly, having drawn on his 1839 book, American Slavery As It Is. Except for publishing Stowe and publicly backing Lincoln, Weld largely withdrew from abolitionist activities after 1844. He lived until 1895, dying at age 92.
Wendell Phillips was the original "limousine liberal", a wealthy attorney who came to the abolitionist movement in 1837 and thereafter became a professional firebrand, joining with Garrison in attacking slavery and actually publicly cursing the Constitution in 1842 in a Boston rally. He remained a red-hot who criticized Lincoln until the Emancipation Proclamation. After the war, he went on to other "progressive" causes: woman suffrage, prison reform, attacking profit capitalism. He died in 1883.
The Beechers, Thad Stevens, and Garrison, everyone knows about.
I'm surprised he wouldn't be in a Civil War Almanac. Several of the prominent politicians of the north were citing his works in their political speeches during the late 1850's.
Here's what a brief web search pulled up on him http://www.lysanderspooner.org/
As for this particular writing, my own take is that it has some interesting parallels with what Alexis de Tocqueville had to say about secession. Tocqueville wrote years before and essentially said that when the union acts to coerce obedience of one of its own, it will have violated the principles of its founding and therefore no longer exist as the union it was created to be.
Spooner's essentially saying 5 years after the war that this happened and the nation that was there before the war is now nothing more than another European style might-makes-right state posing as a principled libertarian democracy. It's inescapably frustration-driven and downright scathing in the harshest use of language imaginable, but that's also a type of flamboyance evident in Spooner and most of his books. In the time since he's become a quasi-icon of the libertarian and anarcho-libertarian types.