Almost all of the Eisner material has been reprinted over the years; your library stystem can probably find several volumes for you. You don't really need to start from the beginning, but you should know in advance that the Spirit's sidekick Ebony was heavily and offensively stereotyped in the early stuff. (Eisner improved him a lot as a result of meeting actual black people in the Army during WWII--he repeatedly apologized over the years for the early Ebony.)