This chapter explores the way modernist picture books reconceive three-dimensionality, and hence the book as object. Around 1900, a Europe-wide vogue for picture books (Gertrud Caspari, Andre Hellé) in which toys come to life overlapped with new enthusiasm for building block play (H. G. Wells and E. Nesbit advocate the construction of “little worlds”), toy-centered ballets, and the explorations of movement and perspective enabled by the advent first of cinema, and then of cubism. The essay also discusses 1920s and 1930s constructivist, cubist, and De Stijl picture books by Lou Loeber, Nathalie Parain, Alexander Rodchenko, and Varvara Stepanova in relationship to the emergence, in schools and art schools, of new art pedagogies centered on paper crafts, a new sense of the picture book itself as a template for future art-making, and of child readers as fledgling artists in their own right.