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.
• The entire work was first published in 1956 and without a copyright notice: see full copy of the same book at http://www.fortran.com/FortranForTheIBM704.pdf or http://www.fh-jena.de/~kleine/history/languages/FortranAutomaticCodingSystemForTheIBM704.pdf for evidence. U.S. works published before 1978 without a copyright notice are in the public domain.
• 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.