This was definitely the Empire targeting or restricting death class professionals, otherwise, why would there not be a single skill book?
Zi Liyuan was able to get his hands on "Skeleton Reinforcement", "Miasma of Death," and "Dark Eye Vision"—three skill books used by death class professionals—simply by a fluke, like a blind cat running into a dead mouse.
"Dark Eye Vision" was luckily obtained by a rookie who had just awakened and changed class, who got extremely lucky in a dungeon and found it in a chest.
As for "Skeleton Reinforcement" and "Miasma of Death," it was pure luck that he encountered a crafty merchant and bought them for a higher price in Dragon Coins than the market rate.
Given that slick merchant Ge Pangzi's slipperiness, it's highly likely that these two skill books were also acquired from rookies.
Therefore, Zi Liyuan didn't even bother to look in the skill book stores set up by the Empire; instead, he went straight to the privately-owned shops.