Highly Recommended
Websites
Robert Sabuda is a pop-up expert who's brilliant website has lots of information about pop-ups including lots of clever pop-up projects from the very simple to the very intricate.
http://robertsabuda.com/popmake/index.asp
Martha Stewart's website has a host of fun card and craft projects
Books
There is a host of books on the subject of paper engineering and pop-ups. Here is some suggested reading - enjoy!
Most are of the books listed are available from Amazon.com
The Pop Up Book by Paul Jackson
Published by Lorenz Books / Anness
Paper Engineering for Pop Up Books and Cards - by Mark Hiner
Published by Tarquin
'Up Pops' Paper Engineering with Elastic Bands - by Mark Hiner
Published by Tarquin
Pop-Up Cards: And Other Greetings That Slide, Dangle & Move - by Sandi Genovese
Paper Engineering and Pop-ups for Dummies - Rob Ives
Pop Up Greeting Cards - by Masahiro Chatani
Published by Ondori
Pop Up Origamic Architecture - by Masahiro Chatani
Published by Ondori
The Encyclopaedia of Origami and Papercraft Techniques - by Paul Jackson
Published by Headline
Origami, a complete step by step guide - by Paul Jackson
Published by Hamlyn
Paper Sculpture - by Tsuneo Taniuchi
Published by Genkosha
3-D Graphics (commercially produced 3-D structures)
A series of handbooks published by PIE books, Japan