Continuing with writing after his retirement, he returned to his interest in tattoos and wrote two books relating to his time in New Zealand, Moko or Maori Tattooing in 1896 and Pounamu: Notes on New Zealand Greenstone.
In the first book, as well as demonstrating and explaining the art of Māori tattooing, he also wrote chapters on the dried tattooed heads or Mokomokai.
 Robley decided to acquire as many examples of Mokomokai as possible, and at length built up a unique collection of 35 heads.
In 1908 he offered them to the New Zealand Government for £1,000; his offer, however, was refused.
Later, with the exception of the five best examples which Robley retained, the collection was purchased by the American Museum of Natural History, New York, for the equivalent of £1,250.

Robley also collected Māori antiquities.
Some of his collection was purchased by the collector William Ockelford Oldman whose collection was purchased by the New Zealand Government in 1948.

Robley maintained a lively correspondence with a number of New Zealanders and maintained close links with New Zealand House during his lifetime.
He died in London on 29 October 1930.
Continuing with writing after his retirement, he returned to his interest in tattoos and wrote two books relating to his time in New Zealand, Moko or Maori Tattooing in 1896 and Pounamu: Notes on New Zealand Greenstone. In the first book, as well as demonstrating and explaining the art of Māori tattooing, he also wrote chapters on the dried tattooed heads or Mokomokai.  Robley decided to acquire as many examples of Mokomokai as possible, and at length built up a unique collection of 35 heads. In 1908 he offered them to the New Zealand Government for £1,000; his offer, however, was refused. Later, with the exception of the five best examples which Robley retained, the collection was purchased by the American Museum of Natural History, New York, for the equivalent of £1,250.
Robley also collected Māori antiquities. Some of his collection was purchased by the collector William Ockelford Oldman whose collection was purchased by the New Zealand Government in 1948.
Robley maintained a lively correspondence with a number of New Zealanders and maintained close links with New Zealand House during his lifetime. He died in London on 29 October 1930.
Horatio Gordon Robley