Psion would be the book I'd think would be of most use to Dredd, since it greatly expands psychic abilities, something quite prevalent in the JD universe.
Dilletante is also quite useful for ideas regarding the idle rich and bored citizens doing something dumb who often crop up in JD. It also has extensive discussion about drugs.
Agent is of use for the espionage aspect of JD. Most of the special spy equipment there will fit right in for Mega City spies, cat burgulars, assassins and such.