If the books aren't canon then why is Fourth Closet Funtime Freddy in ucn for? Why is Twisted wolf in the poster in the fnaf6 ally? Why take details from the books and use them for lore? Or perhaps we're met with this on Fazbear Fright books the same name for the fnaf3 building.
If it wasn't for the books we wouldn't have known about Henry or many details. Heck we wouldn't have known Charlie or Elizabeth's ages or even why William kills. Scott did admit the first trilogy being a retelling of the lore story with different perspective etc.
I will say it here we finally can understand what Shadow Bonnie is as he has a story in Blackbird. Scotts using the books to answer the gaps and old lore we're still questioning. I honestly don't think he's making fnaf games to keep answering old lore/fill gaps. What seems to be his final game projects are extensions leading to Security Breach a remake of fnaf1 which i'm thinking is AAA game, and Fnaf Into Madness.