Outfit your figures with a proper library with this parametric book. Also included are a variety of premade versions- small, medium, and large.
The file is set up to have a cover thickness of 0.8mm, which seemed about right for integration with 1:18 scale figures. If that's what you want, just use the customizer and set the width, height, and thickness values to whatever you want.
If you want to make them fit with other scales, download the scad file and modify the cover_thick variable.