Purgatory calls for attention, but it has a thread that is easy to follow. The climb has stages, the rule of the place is stated in a few lines, and the doctrine of love in canto XVII gives the key to everything that follows. The difficulty lies less in the idea than in the form and the references.
The form is the verse: 33 cantos in terza rima, with about 140 lines each. An annotated edition helps, because the poem cites many people from thirteenth-century Italy and from around 1300 whom today's reader does not know: Manfred, Sordello, Guido del Duca, Forese, Bonagiunta. Politics, which runs from the invective of canto VI to the two suns of Rome, wearies today's reader, and it is what keeps the poem from being mere doctrine.
There are three passages where reading demands the most: the doctrine of love and of free will (cantos XVII and XVIII), Statius's explanation of how the soul is generated (XXV), and the procession of Eden, which the poem describes through colors, gestures and numbers, without giving the names that commentators assign to each figure. Virgil himself sets the limit of reason: "be content, human kind, with the quia."
A practical way to read it is one canto at a time, asking in each one who speaks and what the person learns to want. Because it is poetry, the text works best read slowly.