You are referring to a novel, and not a text book. It is understandable as the conventional bomb itself is more dangerous than the dirty stuff that is added. Please note that a dirty bomb is dirty due to the radioactive material in it. This material is not a nuclear device, it just contaminates the area with the radioactive materials. This contamination will last a only short time, depending on the material, the amount and the particle size.
The danger is due to its psychological effect nothing else.
You say this contamination will last a only short time. However it is my understanding that some suitable elements for a “dirty bomb” have a half life of a number of years, not a “short time.”
Quite correct and shielding necessary to conceal and safely transport such a device would make it prohibitively heavy and large.
Much better bang for your buck with conventional explosives. Don't get me wrong it will make a big mess but that is about it.