The Spirit and Will Eisner's Early Comics
Will Eisner's Later Career and Legacy
Those who only know of The Spirit through the 2007 Frank Miller movie should definitely read up on the real thing. (Though I didn't find Miller's adaptation as bad as everyone said – it just felt way more like Sin City than anything Eisner ever did, with the good humor of the original characters replaced with Miller-esque ultra-violence.)
Those more interested in the technical side of comics, from Eisner's perspective (and others), should check out these Six Classics of Comic Book Theory.