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"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. Getty

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

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