A great essay on it, here.
On another note..
One thing that is interesting is how many social anarchists label Spooner as "anti-capitalist" for some of his criticisms of wage labor (most of these criticisms were discussing wage labor in the context of *state* capitalism, but some were not).
Ancaps Sam Konkin and David Friedman (among others) both made similar criticisms of wage labor, yet neither of them are hailed as "anti-capitalists," like Spooner is. And Spooner's conception of natural rights is basically the same as Rothbard's, which is why I think it's politically accurate to call him an anarcho-capitalist--albeit a "left-leaning" one.
Why do the collectivists want to claim Spooner so badly? My guess that they can't stand the thought of not being able to "have" a famous abolitionist.