The PTC Referencing Style Guide is available as a PDF document (version 1.4, 2017). In addition to a number of minor corrections, this version also contains a brief, new section on academic writing style. The two previous versions (1.2 & 1.3) may also still be used.
Update (13 March 2017): Version 1.4 contains a couple of errors, and also does not mention one particular case (a book written by one person, but edited by another). This will be corrected in the 2018 version. In the meantime, please consult the Corrections and Additions Page (20170313).