If I may, here’s some additions:
theoi.com has an excellent section on the gods of the Underworld (X)
Oxford Classical Mythology Online (the companion website to Classical Mythology by Mark Morford, Robert J. Lenardon, Michael Sham) has material about Demeter and the Eleusinian Mysteries (Chapter 14) and Views of the Afterlife: The Realm of Hades (Chapter 15)
Plato’s The Myth of Er discusses the immortality of the soul and the afterlife. (X)
These articles can be read for free with registration:
Caskey, John L., and Sotiros I.Dakaris. “The Dark Palace of Hades.” Archaeology, vol. 15, no. 2, 1962, pp. 85–93. JSTOR, JSTOR, www.jstor.org/stable/41670313. (X)
Fairbanks, Arthur. “The Chthonic Gods of Greek Religion.” The American Journal of Philology, vol. 21, no. 3, 1900, pp. 241–259. JSTOR, JSTOR, www.jstor.org/stable/287716.(X)
Hewitt, Joseph William. “The Propitiation of Zeus.” Harvard Studies in Classical Philology, vol. 19, 1908, pp. 61–120. JSTOR, JSTOR, www.jstor.org/stable/310320.(X)
Leventi, Iphigeneia. “The Mondragone Relief Revisited: Eleusinian Cult Iconography in Campania.” Hesperia: The Journal of the American School of Classical Studies at Athens, vol. 76, no. 1, 2007, pp. 107–141. JSTOR, JSTOR, www.jstor.org/stable/25068014.(X)
Puhvel, Jaan. “‘Meadow of the Otherworld’ in Indo-European Tradition.” Zeitschrift Für Vergleichende Sprachforschung, vol. 83, no. 1, 1969, pp. 64–69. JSTOR, JSTOR, www.jstor.org/stable/40849412. (X)
Thayer, H. S. “The Myth of Er.” History of Philosophy Quarterly, vol. 5, no. 4, 1988, pp. 369–384. JSTOR, JSTOR, www.jstor.org/stable/27743871.(X)