Complex sentence: The third volume focusses on Demeter, Kore-Persephone, and Hades/Plouton, followed by the fourth volume, which focusses on Poseidon and Apollo.
Split into simpler sentences: The third volume focusses on Demeter, Kore-Persephone, and Hades/Plouton. The fourth volume focusses on Poseidon and Apollo.