The G-man and the Diamond King : a true FBI crime story of the 1930’s / William E. Plunkett. (OCLC #915806586)
RDA A.11.8 on Capitalizaiton of Epithets says to capitalize an epithet occurring with, or used in place of, a personal name, so in this title I’ve capitalized “Diamond King”.
RDA A.28 on Capitalization of Single and Multiple Letters Used as Words or Parts of Compounds says to capitalize a letter that refers to a letter of the alphabet, so I’ve also capitalized “G-man” (short for Government Man, slang or FBI agent).
This makes my title proper:
The G-man and the Diamond King