I'm going to copy my review from The Sinner Book 1, because it is so apt:

I absolutely MUST have a hard copy of this book. It is a necessity for any library that also has Dante's Comedy. This is the only novel that has ever held a candle to Dante Alighieri. I didn't expect prose, but I got it, and I loved it. This is a book I will come back to again and again.

If you like prose (that isn't nearly as inaccessible as Dante) and you love a good fable steeped in religious stories and lore, you'll love this series.