Basic Pop-Up Forms and The Pocket Paper Engineer, Volume 2 Platforms and Props. Volume 3 teaches how to create V-fold pop ups and it's many different incarnations. Definitely a must have for any paper engineer.
You can pre-order The Pocket Paper Engineer, Volume 3: V-Folds here, and you may also want to order Volume 1 and Volume 2 to have a complete understanding of paper engineering. To learn more about Carol Barton and her wonderful pop ups, please visit her site!