Later in the game you can respec the main characters, which would result in any skills they know being unlearned. Since some skillbooks are currently hard (or impossible) to find at higher character levels, if you want the option to be able to respec, it is best to have spare skillbooks for any skill that can be bought or crafted while they are available, rather than try to find them after the fact.
Some skillbooks are unique or the skills only available at character creation.
