However somebody had to write the book. Is there any evidence pointing to who did it?
William S. Sadler (probably).
The book was likely authored (or part-authored) by a doctor named William Sadler, a man who garnered some local fame as a debunker of psychics but latterly formed his own religious cult and became immensely wealthy as a result.
He supposedly spent some time with an unconscious psychic who translated readings from various celestial beings to a court-trained stenographer. Sadler was suspiciously vague as to the identity of this remarkable individual (and the stenographer whose identity he had no reason to conceal) and hence it seems more likely that he wrote the pages himself, noting that he ready access to the various religious books and essays from which the papers were cribbed and was a prolific writer of books of medicine.
Sadler confessed there that he'd been introduced to an individual in the summer of 1911 who was an apparent exception to his thesis, and that he had been present at two-hundred-fifty night sessions recorded by a stenographer: "This man is utterly unconscious, wholly oblivious to what takes place, and, unless told about it subsequently, never knows that he has been used as a sort of clearing house for the coming and going of alleged extra-planetary personalities." The doctor reassured his readers that the message being received was "essentially Christian and is, on the whole, entirely harmonious with the known scientific facts and truths of this age."
In later years his story changed, having the psychic produce papers out of thin air which would arrive at his bedside table. Rather than handing over the original papers, Sadler would read them aloud and the members of the cult would take notes which would then be retyped as pages that eventually made up the bulk of the Urantia book.