I first read Dante's Inferno back in high school when it was assigned to me by an English teacher who decided I needed a challenge, and in the years since then I've read it a few more times (once for a college class, and a couple more times for various research projects of my own devising). But like most people, I never seemed to be able to talk myself into checking out the other two-thirds of the Divine Comedy (1308 - 1321), Purgatorio and Paradiso. I guess reading poetic medieval theology loses its appeal when it doesn't involve demons, grotesque torments, and an infernal hierarchy stuffed to overflowing with Dante's political rivals.
By the end of Inferno, Dante and his guide Virgil (as in the Roman author of the Aeneid) have traveled down through the rings of Hell, passed by Satan in his icy prison at the center of the Earth (yes, the center of Dante's hell is frozen--keep that in mind the next time someone whips out that tired old "when hell freezes over" line), and come out on the other side of the planet to find themselves at the base of Mount Purgatory. Purgatorio takes up the story there, following Dante and Virgil as they ascend the cornices of Mount Purgatory and finishes with them arriving at the Earthly Paradise located between purgatory and heaven. Paradiso then follows Dante and his new guide Beatrice as they ascend through the spheres of the Ptolemaic universe, each of which serves to introduce Dante to a different subgroup of saved souls, and ends with him being allowed to look upon God himself.
I have to admit, it was hard to motivate myself to finish Purgatorio. In many respects, it is the mirror opposite of Inferno. Hell's rings are arranged from the least serious sinners (the lustful) to the greatest (traitors) in Inferno, while purgatory's cornices in Purgatorio are arranged from greatest sin to least; while the damned in hell are trapped against their will for all eternity, the souls in purgatory stay at each cornice of their own free will until they feel they have been purged of that particular sin; and so on. In short, Purgatorio was pretty much everything I feared it would be: a dull, tedious imitation of Inferno, lightened only by sporadic bursts of cattiness directed at various people who had earned Dante's ire.
Imagine my surprise when Paradiso turned out to be deft, innovative, and eminently readable. Granted, it still includes many of the same opaque discourses on the finer points of medieval theology and astronomy, but the rest of the work is fascinating enough to make these digressions bearable. To Dante, heaven exists in an abstract form that is beyond the ability of the mortal mind to comprehend, so what he is shown by Beatrice is an artificial framework used to put a concrete face on a purely ethereal realm. The balance Dante strikes in his poetry between the apparent reality visible to him and the actual reality that lurks behind it gives Paradiso a vitality that helps it to match -- and in some places surpass -- what he was able to accomplish in Inferno.