It seems reviewers tended to like this third book of the series the best. I didn't: here the balance between great and problematic stuff tipped for me. While it does showcase Rosenberg's maturation - primarily of theme and heft - it comes at the price of the manic storytelling joy of the previous books.
Problematic: Ash is more than a bit of a Mary Sue. As is often the case in series books with a Sue-ish lead, the supporting cast is more fun, vivid, and down-to-earth, and serves as an important check on the excesses of the main character. Here Ash is stripped of his support - a solid plot device - but at the price of losing that counterbalance.
"Resisting seduction" continues as a theme, and while the temptations of power are a critical element of the series construction, the extent to which that's repeatedly presented through female sexuality is a bit troubling.
Great: This volume fleshes out the world and magic systems significantly, and it's well done. As usual, the mystery is well-crafted, convoluted without being messy or unsolveable. Ash's sympathy and empathy are a nice change from the more thuggish kick-ass heroes of urban fantasy.
In all, Pantheon is well worth reading, and a solid take on the Persephone story, and I'm eager for more in Rosenberg's series. I just hope the author learns to balance heft and maturity with the fun of the earlier volumes.