Verticality, and related figures such as the tower, stack, or mountain, are commonly used as spatial metaphors to express the hierarchy that we apply to information and knowledge. But these metaphors that transform the vertical dimension of knowledge into words are also translated into library architecture. Different libraries include, or have been built in the form of, a tower. In these cases, verticality as a spatial metaphor is folded back onto the spatial and architectural field where it originated. Library towers transform verticality as a concept that conveys relations in knowledge into architectural language. The translation of verticality as a dimension of knowledge into architecture thus forms a strange double bind between space and knowledge. This article analyzes how libraries have expressed the vertical dimension of knowledge in their architecture and identifies different approaches that make the vertical dimension of knowledge architecturally present. The library of Ghent University (Belgium), by Henry van de Velde, includes a storehouse of books that has been completely accommodated in a tower. The architecture of the French National Library, by Dominique Perrault, plays with the metaphor of the tower in a semantic manner. Other libraries, such as the “Book Mountain” by MVRDV in Spijkenisse, exploit the book stack architecturally; and some libraries, such as The Netherlands Institute for Sound and Vision, by Neutelings Riedijk architects, do not build up but down, in the underground, to house their collections.