Little Green Frogs (Hardcover), by Frances Barry. This book explains the life cycle from egg to frog in simple, toddler-level text. But the unique format of the fold-out pages steals the show. As the reader finishes a page, he lifts it outward, forming an ever-increasing fan shape that, in the end, reveals itself to be a large lily pad. (The author, Frances Barry, is well known for Duckie's Rainbow, which I've seen oft-praised, but haven't yet read). I'm not sure we're likely to find this one in libraries in one piece, but I think it would be great fun while it lasted (actually, we're lucky in that the Infant Bibliophile has done very little in the way of book destruction).
Bookworm's interest at 21 months: He was able to manipulate the fold out pages easily (and paid more attention to those than the text). Folding the book back together wasn't as intuitive, but he did fine with that too.(Apparently, I use parentheses often.)