The bug
When you load a 1.8 world in 1.13 book names disappear and the book says Invalid book tag. This is obviously a problem, especially if there's books in old chunks that haven't been loaded forever. Case in point: https://mcworldmap.com/worlds/eTbLl. On the bright side if you load all the chunks in 1.12 before 1.13 the world smiles at you instead and lets your innocent books live.
Can confirm. I first ran into this issue after opening the 'Map specific rules' book of the well known 'Uncharted Territory 3' map from https://www.minecraftforum.net/forums/mapping-and-modding-java-edition/maps/1515043-amlups-uncharted-territory-series-new-map-in