To become part of what we consider evidence in book history, physical objects (books) must be collected, organized, catalogued, and compared with other books. No one in this process is disinterested: collectors and dealers are concerned with value; cataloguers are concerned with consistency and coherence and they work under severe constraints of time; scholars are concerned with communicating with other scholars. Fifteenth-century books are historically a nexus for all these often competing concerns, and I have focused on these in this study for reasons of pure convenience: fifteenth-century books obviously have a longer reception history than other printed books, and because of their privileged place in the book trade, they tend to be better and more thoroughly catalogued than books of other periods.