Sacramento D.A. Details 'Golden State Killer's Tiny Penis in New Book

"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.

The D.A. says the photog got on his knees to snap DeAngelo's genitalia but "grew frustrated after several failed attempts." Ho says a detective "threw up his hands in the air in exasperation and barked over the intercom, 'There’s nothing there.'"

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.