Rose Jeffries Peebles - The Legend of Longinus in Ecclesiastical Tradition and in English Literature (1911)
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One of the more fascinating Christian legends is that of Longinus, the Roman soldier who pierced Jesus' side then converted and was martyred (despite often being called Saint Longinus he wasn't canonized). There's more story such as being blind then cured, becoming a monk and so forth until getting to a blind woman rescuing the martyr's severed head from a trash heap. All that's in the Biblical canon is an unnamed soldier with the spear - the rest arose over several centuries.
The Legend of Longinus traces the story from apocryphal texts through Church fathers, martyrologies and other sources. As the full title says there's a focus on English literature though it mostly stops after the middle ages but I'm not clear whether there are fewer references or if that's just the author's end point. A final couple of chapters touch on connections to Celtic myth and the Grail legends.
The book was Peebles' doctoral dissertation. She later became the chair of Vassar's English department where she was Elizabeth Bishop's favorite professor.