There aren't many books or poems that need (or deserve) another book to explain them, but Dante's Commedia is one, and this is an excellent guide.
Reading Dante isn't the usual guide book, though. Instead of following the structure of the poem, it picks out a few themes (love, power, the use of language) and traces them through the complete hierarchy of hell, purgatory, and heaven. Along the way it provides a vital guide to the social and political context of 14th century Italy and some sights in Florence to see and relate to Dante's time. Linking to one of my other great loves, it's also full of illuminated letters and images used in the various editions of Commedia (although sady not in colour). For someone like me who reads Dante in translation, it both highlights the effects that only come through in the original, and acts as a spur to learn Italian if for no other reason than to enjoy Dante more.
One other thing that I think I should note is the physicality of the book itself. It's beautifully presented, with deckle-edged (unevenly torn) pages, and typeset very sympathetically. Altogether a delight to read, although often quite intense.
Finished on Sat, 11 Apr 2015 09:20:16 -0700. Rating 5/5.