One of the most interesting sources of information on parallel manipulators is this website:
http://www.parallemic.org/
This site has been created by professor Ilian Bonev of the ETS in Montreal (Ecole de Technologie Superieure - Superior Technology School) while he was a PhD student with the very renowed professor Clement Gosselin.
I agree that this is the most comprehensive on-line resource in the field of parallel robots (hexapods, Gough-Stewart platforms, parallel kinematic machines, motion bases, etc.).
In order to obtain a good idea on references on parallel robots, I rely on a quite large (but not necessarily complete) source of information: Jean-Pierre Merlet's website:
http://www-sop.inria.fr/coprin/equipe/merlet/merlet_eng.html
Luc