If you buy the skill, you get a lot for the buck. Others require you to specify one item only, like advanced education and combat type skills. For skills with prerequisites, the skill level cannot exceed the prerequisite's skill level. For example, if your basic skullduggery is level 3, your advanced skullduggery cannot exceed level 3. A very important skill is Arcane Lore.
Arcane lore offers bonus points to defense rolls against magic type attacks.