The Walking Library, inaugurated in 2012, has functioned as a mobile laboratory and art project for the ongoing exploration of the relationships between environments, books, reading and writing. In this essay, our focus turns to The Walking Library’s function as a library, asking: ‘What sort of library is a walking library? What does a walking library do—for its books and its borrowers and the places through which it moves? And what can it reveal or teach us about libraries, books, reading and environment?’ In a context in which data has become ‘mobile’, we explore the mobility of physical books through the Walking Library’s social and architextural designs and structures. The book on the move is recognised as the material of social bonding. The Walking Library depends upon and promotes the mobility of books through social networks by gifting, lending, borrowing and sharing; it is the social capacity—the social capital—of The Walking Library, and of walking and reading together, which concerns us most here. The Walking Library has offered temporary spaces for sociality, for shared contemplation, poetic spatiality and kinaesthetic comprehension. In doing so, it has generated a heightened sense of books’ sociability, spatiality and mobility through a stronger understanding of the inter-dependencies of reading, walking, time and place.