→IEC-standard

standard IEC 61131-3 Edition 3.0, "Programmable controllers – Part 3: Programming languages"; basis for a standardized programming for PLCs considering the modern concepts of software technology, International Electrotechnical Commission; edition 3.0, 2013-02

If you are interested in books on the IEC-standard, see PLCopen website http://www.plcopen.org/pages/promotion/iec_books/ to find a list of recommended books.
If you are interested in the IEC-standard, see PLCopen website http://www.plcopen.org/pages/promotion/iec_standard/ to find links where you can order the standard.