|My photo of one of the yews at St. Mary's Cusop, Herefordshire on the border with Wales.|
My latest dive into yew lore is The Ancient Yew: A History of Taxus Baccata, by Robert Bevan-Jones (Oxbow Books, Oxford 2017). The author is amazing in his comprehensive assembly of the evidence. I think it is all there as well as can be gathered. There are extensive, footnoted sources. However, the writing style is a bit jumpy as is the presentation of the evidence. Bevan-Jones is strongest in his theme that the churchyard yews are "at least" 1500 years old which places them right at the time of the establishment of Christianity in Britain by the Celtic Saints. He first discusses the botanical challenges of the strange growth patterns of the tree and then presents the best estimates by charts and graphs to support his Celtic-Saint-planting theory.
But then came the Iceman. In the final chapter, "Yew: an Archaeological Perspective" (which perhaps should have come earlier at least in a chronological sense):