He has written extensively on the Celtic Saints and composes his own Celtic prayers. His book, Border Lands, is sub-titled "The Best of David Adam's Celtic Vision".
He starts out by discussing the "marginal lands" of Holy Island and goes on to talk about the fact that we often live on "the edge of things" and that this experience is a call to explore and experience all that God has for us to see and do. In the introduction to the book, he quotes T.S.Eliot's Little Gidding:
And the end of all our exploring
Will be to arrive where we started
And know the place for the first time."