This work is in the public domain in the United States because it was legally published within the United States (or the United Nations Headquarters in New York subject to Section 7 of the United States Headquarters Agreement) before 1964, and copyright was not renewed.
Works published in 1961 would have had to renew their copyright in either 1988 or 1989, i.e. at least 27 years after they were first published/registered but not later than 31 December in the 28th year. As this work's copyright was not renewed, it entered the public domain on 1 January 1990.
This work may be in the public domain in countries and areas with longer native copyright terms that apply the rule of the shorter term to foreign works.