As for actual supplements, then Scrolls of Skelos because Xaltotun's stats are there, as is the Heart of Ahriman. Aquilonia, because that is what is at stake. The article on Khemi in Signs and Portents, because it has Akivasha's corrected statistics. Road of Kings, because the trail goes all over the place. Messantia, because part of the adventure passes through there. Pirate Isles because of the sea part of the adventure. Hyboria's Fiercest just because. The upcoming sourcebook on Stygia might be good.