Ah well, that's interesting and curious. I can't say I've had a particular problem with items poking into my back with my better quality backpacks. I find the padding on my packs (I used to use MEC packs and now Tom Bihn) has been more than sufficient and I studied three degrees worth of engineering so I've carried my fair share of books and odd items, including exercise gear and bulky lunches.

In fact, I find find some of the newer packs place the laptop away from the back panel. I believe the thinking is that since the back has a natural curve to it, placing a large, rigid item will not allow the pack to conform to the back easily; smaller items that can shift a bit will move under compression to fill in voids and thus better align with the curve. This might be a little less protection for the laptop but it's the tradeoff between ergonomics and protection. I'm not sure how rigid your box is and if that affects anything.

As for distributing the weight from left to right, I guess a box would help, but would you still not have to somehow secure the load itself within the box from sliding from side to side if it isn't full?

Worn edges? Seriously? Worn edges are good! That means the books have seen good use and that you're massively educated!