I like your idea, but let me ask one thing, why trapdoor? I see, what you're going for, but why not using wooden pressure plates, or slabs (any slim wooden "plank item", ok slabs aren't exactly slim, anyway) instead of doors? Sure when you craft trapdoor, you get two pieces at once, so it's actually convenient, but still using door for a book cover, I personally find it kinda unnatural (but honestly what is in Minecraft natural?). I use trapdoor for building my bases, so wasting them for books, isn't quite my cup of tea.