It’s subtle, but I’m worried it’ll effect the look of the book later on when I apply textures etc.
Is there a way to more effectively fill the area between vertices, or is my method (selecting verts and f key) just fine and the “distortions” won’t show up in the render?
Its because you have shade smooth turned on… try in the mesh properties tab (in between the material tab and the modifiers tab) enabling auto smooth and changing the angle to something like 50 (play around with it)
I prefer to do it in the mesh properties tab instead of the modifier tab… they will end up with the same result… the only difference is that one does not use modifiers, and if an object uses modifiers it cannot be instanced properly.