Most books on Celtic Christianity fall into two categories. Many are cloyingly simplistic efforts to breathe new life into the belief and worship patterns of believers that are burned out on mainstream Christianity. Most of the rest are New Agey-type musings attempting to couple Christianity with some presupposed reconstruction of murky Druidic belief.
This is the only book to date that I have come across that told me something convincing about the impact of traditional Celtic belief on Christian worship. Upon reading this book it became clear to me how questions of liminal belief and the place of worship being the center of the world are so deep seated in the Celtic Christian perspective that you can overlook their significance. This and the in-depth treatment of the Celtic understanding of pilgrimages were very enlightening.
This is a deceptively short book; I must admit that I've read it several times, and always keep coming back to it for reference, which is something I can't say about many books on the subject. Highly recommended.
Sheldrake has produced one of the more rich and historically accurate works on Celtic-Christianity to date. I enjoyed very much his working with various themes inseparable from Celtic spirituality, such as liminal space, pilgrimage and thresholds. I also found a great deal of value in his rekindling an awareness in the reader of the Rule of Tallacht, which includes a prayer-trance posture used by Celtic monks that undoubtedly hails from the druidic traditions. I thought he did a very good job of conveying that the native Druidism of Ireland and Scotland was not simply replaced over night by Celtic-Christian mysticism, but rather that Celtic-Christian mysticism grew out of Druidism. A good book.