“The Golden State Killer” was brought to justice in part because of a distinctive micro-penis … and a new book about his capture details just how small his penis is … smaller than a pinky finger.

TMZ obtained a copy of Sacramento County District Attorney Thien Ho‘s new book, “The People vs. the Golden State Killer” … and there’s a big focus on Joseph DeAngelo‘s small penis.

Ho is writing about law enforcement obtaining circumstantial evidence to corroborate victim testimony about DeAngelo’s micro-penis and he recalls how a police photographer was instructed to take photos of DeAngelo’s penis from multiple angles.

Ho says DeAngelo was directed to “spread his legs and pull back the foreskin” … and when the photog got the money shot, it proved just how small DeAngelo’s manhood was … “smaller than the circumference of a dime and its length is equal to the tip of your pinky.”

DeAngelo terrorized California in the 1970s and ’80s, murdering 13 people and raping 50 in a series of horrific crimes … but he wasn’t busted until April 2018 after being identified by crime-scene DNA.