Jie Qi’s Electronic Popables feature some interesting uses for flexible circuitry in a pop-up book context –

Electronic Popables is an interactive pop-up book that sparkles, sings, and moves. The book integrates traditional pop-up mechanisms with thin, flexible, paper-based electronics and the result is a book that looks and functions much like an ordinary pop-up with the added element of dynamic interactivity. Electronic Popables was built by Jie Qi, with assistance from Leah Buechley and Tshen Chew.