The basic problem is that books are organic, explosives are organic. Their current x-ray can determine that something is organic but can't differentiate books from a block of explosives.
For many years a stack of books would get your checked bag a TSA inspection. They're just getting more obsessive about this.
Note that the same thing applies if you're getting an agricultural inspection--their x-ray actually only detects whether something is organic and food-type density. Pack all such material together and you minimize the amount of suitcase pawing they do. (Coming from China agricultural inspections are very common for us.)
Explosives may or may not be organic. You realize that's not how x-rays work, right? I mean, seriously. Trust me, airport screens will find hidden metal guns, knives, etc.