
Extract the answer to the question from the following context.
Question: What clue words would indicate that Hayek previously wrote a different book?
Context: From 1962 until his retirement in 1968, he was a professor at the University of Freiburg, West Germany, where he began work on his next book, Law, Legislation and Liberty. Hayek regarded his years at Freiburg as "very fruitful". Following his retirement, Hayek spent a year as a visiting professor of philosophy at the University of California, Los Angeles, where he continued work on Law, Legislation and Liberty, teaching a graduate seminar by the same name and another on the philosophy of social science. Primary drafts of the book were completed by 1970, but Hayek chose to rework his drafts and finally brought the book to publication in three volumes in 1973, 1976 and 1979.

on his next book
