This is an odd quirk in the design, resulting from two somewhat contradictory ideas:
First, they want you to have the freedom to build your character the way you want... but they also want some skills to be extra special and so are only available at character creation or from one spellbook in the game.
I'm not a big fan of the one-time only skillbooks, in part because they will be lost if you respec, but mainly because most of the time, those one-off skills are actually not all that fantastic in the first place. A lot are just minor upgrades to an existing skill you can get anywhere.
