A valuable resource for people who love people who love pop-up books.
http://www.movablebooksociety.org/

I started out working for this guy. Robert is arguably the most influential paper engineer working today.
http://www.robertsabuda.com/

His long-time partner, Matt is no slouch either.
http://www.matthewreinhart.com/

Another emerging artist from their studio:
http://www.kyleolmon.com/

ditto
http://www.patriciafrybooks.com/

A good place (besides ebay) to find rare books:
http://www.abebooks.com/

A nicely designed site dedicated to pop-up book criticism in French:
http://www.livresanimes.com/