Thompson wrote many letters, which were his primary means of personal communication. He made carbon copies of all his letters, usually typed, a habit begun in his teenage years. The Fear and Loathing Letters is a three-volume collection of selections from Thompson's correspondence, edited by the historian Douglas Brinkley. The first volume, The Proud Highway was published in 1997, and contains letters from 1955 to 1967. Fear and Loathing in America was published in 2000 and contains letters dating from 1968 to 1976. A third volume, titled The Mutineer: Rants, Ravings, and Missives from the Mountaintop 1977-2005 was edited by Douglas Brinkley and published by Simon & Schuster in 2005. As of January 2018, it has yet to be sold to the public. It contains a special introduction by Johnny Depp.
Question: How many years does the Proud Highway contain letters from 1955 to 1967?
Answer: 12