The Englishman Who Posted Himself and Other Curious Objects by John Tingey, published in 2010, is a fascinating study of the creative challenges W. Reginald Bray (1879-1939) set forth to the Royal Mail during his lifetime.
His experiments began in 1898 after obtaining a copy of the Post Office Guide, regulations published by the British postal authorities. It seems he considered these regulations a challenge, not a guideline. This enjoyable read includes plenty of photographs to illustrate his many and varied postal antics.
He may well be the pioneer of mail art. I wonder, did the author of Flat Stanley know of this Englishman who traveled by registered post in 1903?