Basically, all of the Enhanced Skill Books were extremely popular, besides the skills that weren't used frequently.
Wealthy people could buy henchmen, followers, women and Epic equipment. However, there were two things that couldn't be bought with money. These were Skill Points, because they were untradeable, and Enhanced Skill Books. This was because there weren't many people capable enough of finding them.
There were many mercenary groups that were extremely powerful and could achieve everything. Even if it was the Molten Core (although obviously not on Nightmare Difficulty), they would be able to bring you along and complete the dungeon. However, it was almost impossible for them to find an Enhanced Skill Book, even with a time frame of half a year.
