How should resources be properly handled in an RTS?

For example, a resource can exist in different states: dropped in the world, stored, or being carried. Then there is also the need for some mechanism to reserve a certain number of resources for a specific unit or use.

I can foresee this being a tricky system to design and implement, so it probably requires some strong upfront planning.

I would greatly appreciate any advice or reading material anyone can share on the subject.