For archival purposes, paper books are actually excellent. The data and the reading mechanism are one and the same.
Digital books are dependent on the devices and data formats that hold them. In thirty years, the books stored on a contemporary nook or kindle may become as inaccessible as a WordStar file stored on a 5 1/4 inch floppy.
Also, when planning for the future we really must take the zombie apocalypse into account, no? If the public power grid fails paper books are going to see a huge spike in popularity.
