For the stacked-books effect, most sites warn that it will take more books than you think it will. Luckily, in our house, enough books is not even a little bit of a problem. Notice the still-full floor-to-ceiling bookcase behind the tree. That's not the only still-full large bookcase in the house, either.
(click to enlarge) I have no idea how many books we actually ended up using, but much of the inside of the tree is filled with books as well, to give it support. I was relieved to discover that a book I wanted to lend to a friend was one of those not in the tree, so I didn't have to a) find it in the spines-inward tree, or b) figure out how to extract it.
I'm not looking forward to the post holiday season when I will have to dismantle the tree and put away all of the books exactly where they belong (when you own this many books, they don't go on shelves willy-nilly, or you'd never find them), but for now I can enjoy my tree made of very processed trees.