Public domain for 3 reasons, as a work first published in the United States:
The book cover, by itself, contains only simple text and simple geometric shapes, below the threshold of originality for copyright protection in the United States.
In any case, http://comminfo.rutgers.edu/~lesk/copyrenew.html shows no record of renewal; earliest Fortran book renewed appears to be Programmer's manual, Fortran II by T.E. Bradshaw (registration A343960 on 1958-05-12, renewal RE-279-786 on 1986-01-06). U.S. works before 1964 must be renewed within 28 years; a 1956 publication without renewal would have entered the public domain by 1984 at the latest.
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 (50 p.m.a.), Mainland China (50 p.m.a., not Hong Kong or Macao), Germany (70 p.m.a.), Mexico (100 p.m.a.), Switzerland (70 p.m.a.), and other countries with individual treaties.
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.