If you use today's HSSP facilities we would appreciate if you would cite this article, which is:
A series of PDB related databases for everyday needs.
Wouter G Touw, Coos Baakman, Jon Black, Tim AH te Beek, E Krieger, Robbie P Joosten, Gert Vriend.
Nucleic Acids Research 2015 January; 43(Database issue): D364-D368.
(PDF).
The original HSSP article and the first article about HSSP file distribution are available. Especially the NAR article about distribution is no longer up-to-date; it is merely listed here for completeness.