I would suggest that to learn all basic command and how they are categorized, u can use any of Redhat bibles or you can use Redhat document RH-033. Warning: These documents and books are highly Redhat dependent. But you can get a very clear Idea of how commands are divided and how can you use them to your advantage.Once you are familiar you can use Running Linux, O Reilly Publications. Anyway it will take quite some time to understand the entire command structure.
-Anuz
