This is a great question. The dos-a-dos can be two volumes of the same text (as books were often printed in multi-volume sets), but they might also be favorite sections of a Bible or verses, favorite poetry, or two related works by different authors (important almanacks, for example). Dos-a-dos were usually commissioned, because they require a bit more finesse, so a book owner would have likely brought whatever they wanted bound in to the binder, and that realm of discretion makes for mixed results
Are dos-a-dos books typically multi-volume sets?
This is a great question. The dos-a-dos can be two volumes of the same text (as books were often printed in multi-volume sets), but they might also be favorite sections of a Bible or verses, favorite poetry, or two related works by different authors (important almanacks, for example). Dos-a-dos were usually commissioned, because they require a bit more finesse, so a book owner would have likely brought whatever they wanted bound in to the binder, and that realm of discretion makes for mixed results