All 5 books share one texture in a placeholder colour. I put it in a custom folder since you need the atlas anyways. There is a palette with the placeholder colours to replace (so the pages still stay the same no matter what), and 5 palettes for the five books. The atlas file uses these to generate the textures. I then make the item model reference those textures instead, and also overlay any overlays I want; a new quill that isn't copied from vanilla, and also a new sash which contrasts the red book with lapis and diamond.

I hope this can demonstrate the occasional use for this new feature! This means if you want to change shape or detailing for all the books at once, it's just one texture (excluding the sash), and if you want to recolour just one, it's easy to make sure you replace everything. I tried to pick something where some stuff is replaced, some isn't, some things are overlayed, and also you can get a variety of palettes, one including an interrupting contrast, and another using the same colour twice on the palette. =]