A search was made from the copyright renewals in 1972[1][2] and the first half of 1973[3] (but admittedly not the second half of 1973[4] because it is not organized in an easily searchable manner) under the titles "A.P.A", "American Psychological Association", "APA", "Duncker", "On Problem Solving", and "Psychological Monographs" - no entries were found. This book appears not to have renewed its copyright.
Note that it may still be copyrighted in jurisdictions that do not apply the rule of the shorter term for US works (depending on the date of the author's death), such as Canada (70 years p.m.a.), Mainland China (50 years p.m.a., not Hong Kong or Macao), Germany (70 years p.m.a.), Mexico (100 years p.m.a.), Switzerland (70 years p.m.a.), and other countries with individual treaties.