Baptism for the dead and the whole LDS concept of the redemption of the dead is one of the most controversial and misunderstood LDS doctrines, and yet one of the most beautiful and philosophically satisfying parts of the LDS experience, in my opinion.

In addition to being evidence of God’s love and fairness, and of the unlimited and majestic power of the Atonement of Jesus Christ, it also provides important evidence for the basic LDS claim of a divine Restoration. It is a topic worthy of careful study.

For a deep dive (or should I say immersion?) into the topic of baptism for the dead, here are some resources that I find helpful:

• "Baptism for the Dead in Early Christianity" by John A. Tvedtnes, from The Temple in Time and Eternity, pp. 55-78. Excellent survey of related practices and teachings that we know of from several early Christian groups.
• "Baptism for the Dead: The Coptic Rationale" — a scholarly review by Dr. John A. Tvedtnes of the early Christian practice of baptism for the dead, as understood by Coptic Christians. This paper was presented at a 1981 symposium in Jerusalem, sponsored by the L.A. Mayer Memorial Museum of Islamic Art and the Israel Ministry of Education and Culture and later published in Special Papers of the Society for Early Historic Archaeology, No. 2 (September 1989). The Jerusalem symposium marked the opening of an exhibit of Coptic art at the museum, where Dr. Tvedtnes one of two American scholars invited to speak.
• Baptism for the Dead in Ancient Times — an early, groundbreaking article by Dr. Hugh Nibley. More evidence has been found since this scholarly exploration was first published in 1948, but it's still valuable today. Nibley provides many exciting leads and reviews a variety of ancient sources not available to Joseph Smith.
• "Does the Bible Teach Salvation for the Dead?" — a review of an anti-Mormon publication by the inimitable John Tvedtnes in FARMS Review of Books, Vol. 10, No. 2, 1998.
• The Apostolic Fathers comment on Christ's visit to the Spirit World — a page at the Chapman Research site.
• "Temple Imagery in the Epistles of Peter" by Daniel B. McKinlay, an excellent and scholarly essay that includes treatment of passages in Peter about the preaching of the Gospel to the dead (including a discussion of alternate interpretations that others have offered). Available at FARMS, reprinted Temples of the Ancient World: Ritual and Symbolism (Salt Lake City: Deseret Book and FARMS, 1994, pp. 492-514).
• Views of a Non-LDS Scholar, Krister Stendahl, on the Ancient Practice of Baptism for the Dead — his short article is on the lower half of the page. Bishop Stendahl, a Lutheran, was formerly the dean and professor of divinity at the Harvard Divinity School.

Finally, you may wish to consult my own LDS FAQ page on baptism for the dead at JeffLindsay.com.