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.