a good, although somewhat superficial comparison of different conceptions of the underworld and the deities inside it.
wish the author wouldve ventured into the metaphysical status of the underworlds more. for instance, the river lethe (oblivion) that has to be crossed to reach the (greek) underworld is brought up, but not fully grasped: behind lethe lies aletheia, the plain of truth (non-oblivion) - the worldly realm is subject to time, but the underworld isnt. living beings, upon birth/reincarnation, forget the eternal truths the souls (and gods) know, which is why, as mentioned in the book, summoned souls of the deceased can act as oracles. this is also how human oracles and poets worked, by engaging in a set of practices that made them immune to lethe's effect (namely e.g. washing oneself in the waters of mnemosyne [memory]). sources for this are detienne's "the masters of truth in archaic greece" and vernant's "myth and thought among the greeks".
wouldve also liked hekate to be covered, which is alluded to but then forgotten about (for anyone interested, a good work on her seems to be johnston's "hekate soteira").
i greatly enjoyed the perennialist framing of entities vs deities: basically entities are noumenous, deities are their culture-informed phenomena.
in the ritual section, the author recommends utilising dark ambient music - if you are wondering why that works so well to breach the veil check out demers' "drone and apocalypse" (a really beautiful, precious book).