This is a thorough historical work and will clearly last as an outstanding study of this subject, fully deserving to be described as definitive (with five stars!). Despite the outstanding nature of the work, however, I do not feel that Jeremy Harte has completely made the case against the "myth" (his word) that holy wells are a relic of "ancient cults against which the church preached in vain ... an important part of the religion of the ancient Celts". As he demonstrates, many wells have a documented medieval history of growing cultic status, or are of recent invention, but of course absence of evidence of a pre-Christian or Celtic origin is not evidence of absence. And suggestive evidence is surely not completely absent. As the author shows, Wales and south-west England have the greatest density of examples, and the east generally the lowest, and the church did have to struggle against unofficial water cults. But I don't want to dwell on that. He convincingly explodes some elements of the "myth".
This is THE book on the subject, although restricted to wells labelled "holy" and with saints names, and excluding Cornwall. As a clever way of keeping down the cost of a scholarly work of restricted appeal, the second and third volumes are supplied as pdfs, with a spreadsheet of the wells, on a CD with the printed volume 1.
Just to add a non-well but related story, I recently visited the ancient sites at Tara, in Ireland. A weather-beaten hawthorn on the site was draped with rags and other baubles, surely a continuing relic of ancient reverence? But no, the guide told me this habit had only started recently.