Translation
The Hollander Translations of the Commedia: I'm biased in favor of the closest person I've had to my own Virgil, my former undergrad professor. While his translation (a collaboration with his wife Jean) itself is not as explicitly poetic as some, it has its own subtle beauty, and is as faithful as they come to the Italian. And, you will not find a more comprehensive nor more helpful, more comprehensive, or more clearly written collection of notes and commentary from a master teacher and scholar.
Helpful Introductory Works
Dante: A Life in Works by Robert Hollander Out of print, but well worth digging for in your local library. By far, the best overview and introduction, not just to Dante's biography and works, but also to his times and poetic/theological/political concerns.
Danteworld's: A Reader's Guide to the Inferno by Guy Raffa: Haven't yet read this one, but have heard amazing things, particularly from first-time readers who found it immensely helpful. Raffa also maintains an invaluable web resource of the same name.
The Cambridge Companion to Dante ed. Rachel Jacoff et al: More available and affordable than Hollander, but with an essay by him and other great Dantisti, this provides a great overview to different facets of Dante's poetic existence.
The Poet's Dante: Twentieth Century Responses ed. Peter Hawkins: A fascinating collection that includes reflections by Eliot, Yeats, Auden, Heaney and other poetic greats of the last century. It's less an introduction to studying Dante, as it is an invitation to fall in love with him.
Dante's Inferno: A New Commentary by Nicola Fosca: Will be released just in time for Lent! Hollander has told me he thinks this is the next great work on Dante; I'm hoping to check it out, and will keep you posted on whether I agree with him!
Dante: Poet-Theologian?
This is my passion in the world of Dante studies. I won't include descriptions, but if you're interested in going deeper into both Dante's utterly unique theology and his revolutionary poetics, I've found these to be amazing reads in their own right.
Dante: Poet of the Secular World by Erich Auerbach
Dante: the Poetics of Conversion by John Freccero
Dante's Testaments: Essays in Scriptural Imagination by Peter Hawkins
Dante and Augustine: Linguistics, Poetics, and Hermeneutics by Simone Marchesi
The Metaphysics of Dante's Commedia by Christian Moevs
Dante's Commedia: Theology as Poetry ed. Vittorio Montemaggio
Helpful Web Resources
Danteworlds
Princeton Dante Project
Dartmouth Dante Project
