Spellbooks store an enormous amount of data, and it's very frustrating to lose all of that from some freak accident or lag or something. My suggestion is to make spellbooks immune to fire (like netherite tools) and ignore them when picking items to splat for GTP.

Alternative, add a pattern that allows you to copy a spellbook in your offhand to an item entity of a spellbook. It makes alot more sense, and has some other uses as well