The author (Michelle McNamara) and one of her editors dubbed the East Area Rapist / Original Night Stalker the Golden State Killer.
He wasn't known as the latter until recently.
The EAR cases weren't even officially tied to the ONS cases until 2001 or so. Multiple cases, multiple nicknames, because nobody realized they were all the same dude. That's a big reason the crimes didn't get more attention, in McNamara's opinion. So she "renamed" him in a huge piece she wrote for Los Angeles magazine in 2013.
So the stalker/killer was known by most people, especially locals, police and media — until recently — as Original Knight Stalker, or East Area Rapist, or EAR/ONS (after the two were officially connected) or the Visalia Ransacker.
The "Golden State Killer," as we known him today, didn't "exist" as a complete suspect before 2013. I suspect that's why she didn't use GSK in the book/story timeline until he actually "became" GSK.