Last Enemy — H. Beam Piper

Part of H. Beam Piper's popular Paratime series, "Last Enemy" follows the work of researcher Dallona of Hadron, who is attempting to determine whether any aspect of the mind, body or soul survives the process of death. However, when her experiments suggest that sentience does outlast the physical body, and she uncovers a link between this immortality and time travel, Dallona is faced with unforeseen consequences she never expected.