I took an interest in pop-up books (and papercraft in general, particularly "urban paper" toys), about a year ago. I got a few books from the local library to see what was available. One of these was The Elements of Pop-Up, and it was, both literally and figuratively, a real standout. I bought a copy immediately.
It's one thing to show diagrams and photos of example structures. It's quite another to have a physical example of those structures right in the book. The value of this is inestimable. Even better, the book gives a URL where you can download PDF files of templates of all the examples in the book so you can print them out and build them yourself.
I've read maybe a half-dozen books on pop-up construction (I'd also recommend The Pop-Up Book: Step-by-Step Instructions for Creating Over 100 Original Paper Projects), and while reading and looking are helpful, nothing beats doing. One simply has to try to make stuff in order to properly understand what's going on, what works, and what doesn't. The Elements of Pop-Up really gives one a leg up on this.
What's really nice is that in addition to examples of basic pop-up structures the book also demonstrates some mechanical actions, such as flaps and spinners.
If you are at all interested in actually making pop-up cards or books, this needs to be one of your first acquisitions.