As far as I know, per today the only 2e book which has really touched on the Darklands setting is the backmatter article "Return to the Darklands" of Book 3 of Sky King's Tomb - Heavy is the Crown. Technically that book was released pre-remaster, but it is the one that starts with the admission of "No we made it all up" from the Pathfinder Society when it comes to old lore about the Darklands and it includes all the new Remaster terms for old races such as Hryngar instead of Duergar, and mentions some of the old drow cities as being old, abandoned, and haunted Ayindilar (cavern elf) cities.