What makes it so is the wonderful, moving, tragic story "The Piper at the Gates of Dawn" (which was originally published independently in "The Custodians"). We follow Old Peter the Tale-Spinner and young Tom (whose magical piping is a legacy of the wizard Morfedd) in their journey to York through a country oppressed by a cruel, militant church. What happens there will change the world forever.
Highly recommended to anyone with an ounce of humanity.
Now segue 1,000 years into the PAST (before the great "Drowning") into a world of cathode ray encephalographs, sine wave frequency, and O.O.B.E.'s (out of body experiences). Seems some British scientists have made contact with Thomas of Norwitch, who is floating in the Somersea (which used to be Somerset) thrown overboard and left for dead because he's a kinsman. The scientist Carver finds himself in Thomas' consciousness. Carver wouldn't be detected at all if not for Jane, the Kinfolk girl with e.s.p. powers who knows he is there and reaches back 1,000 years to contact the Bride of Time in his reality.
At first you might get the impression that this is sword and sorcery at worst or fantasy at best until you get to the second book and you know it's true science fiction. I know of few writers who can pull this off - Ursula LeGuin comes to mind. This novel, because of the "Church Militant", has been compared to *A Canticle for Libowitz*, but for sheer lyricism and philosophical depth, it reminds me most of *Engine Summer* by John Crowley. Cowper made this into a trilogy - the kinship novels - all well worth reading, but this first one, with its sense of wonder and delight, is, in my opinion, his best.