Book 3. Serious spoilers for the first two ahead. . . .
It opens with discussion of the missing keys to the collar. Lexi's idea to go search the ruins for the original is shot down, hard, but when Hades leaves and doesn't come back, Jake and Lexi set about searching for the expert who can tell them how to replace it: the dead Hephaestus. Just a little jaunt across the landscape, including the rivers.
It involves a baseball cap, godly gaps in memory, Cerberus's dubbing Lexi "Bossy Girl," a nasty bargain by the nymph Styx, the rule about gods and temples, finding your way about, harpies who won't let anyone near Tartatus, and more.