In order to add the in-game meshes and textures to an item I'm creating, do I have to unpackage the .bsa files into my Data folder so I can access them directly? Or do I have to do the roundabout way of copying a previous item and changing all its properties?

Say for instance I wanted to create an in-game book with the same world art as the Book of the Dragonborn. How would I go about adding the exact mesh to the book I created? The "World Art" space is greyed out and I can't type the directory there.

I haven't seen it yet (just logged in), but I'm pretty sure it works like in previous kits; if you change an id it will ask to create a new instance. So just open a book you wish to use as a cover, change an id, save and make a new object.