If I may, hereβs some additions:
theoi.com has an excellent section on the gods of the Underworld (X)
Oxford Classical Mythology Online (the companion website toΒ Classical MythologyΒ by Mark Morford, Robert J. Lenardon, Michael Sham) has material about Demeter and the Eleusinian Mysteries (Chapter 14) andΒ Views of the Afterlife: The Realm of Hades (Chapter 15)
Platoβs The Myth of ErΒ 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. βThe Dark Palace of Hades.βΒ Archaeology, vol. 15, no. 2, 1962, pp. 85β93. JSTOR, JSTOR, www.jstor.org/stable/41670313. (X)
Fairbanks, Arthur. βThe Chthonic Gods of Greek Religion.β The American Journal of Philology, vol. 21, no. 3, 1900, pp. 241β259. JSTOR, JSTOR, www.jstor.org/stable/287716.(X)
Hewitt, Joseph William. βThe Propitiation of Zeus.β Harvard Studies in Classical Philology, vol. 19, 1908, pp. 61β120. JSTOR, JSTOR, www.jstor.org/stable/310320.(X)
Leventi, Iphigeneia. βThe Mondragone Relief Revisited: Eleusinian Cult Iconography in Campania.β Hesperia: The Journal of the American School of Classical Studies at Athens, vol. 76, no. 1, 2007, pp. 107β141. JSTOR, JSTOR, www.jstor.org/stable/25068014.(X)
Puhvel, Jaan. ββMeadow of the Otherworldβ in Indo-European Tradition.β Zeitschrift FΓΌr Vergleichende Sprachforschung, vol. 83, no. 1, 1969, pp. 64β69. JSTOR, JSTOR, www.jstor.org/stable/40849412. (X)
Thayer, H. S. βThe Myth of Er.β History of Philosophy Quarterly, vol. 5, no. 4, 1988, pp. 369β384. JSTOR, JSTOR, www.jstor.org/stable/27743871.(X)