The Jargon File (version 4.4.7, 29 Dec 2003):

Purple Book
 n.

    1. The System V Interface Definition. The covers of the first editions were
    an amazingly nauseating shade of off-lavender.

    2. Syn. Wizard Book. Donald Lewine's POSIX Programmer's Guide (O'Reilly,
    1991, ISBN 0-937175-73-0). See also book titles.


The Free On-line Dictionary of Computing (30 December 2018):

Purple Book

   1.  The "System V Interface Definition".  The
   covers of the first editions were an amazingly nauseating
   shade of off-lavender.

   2.  The Wizard Book.

   See also book titles.

   [Jargon File]