FNAF has been my favorite, these books will creep you out and fill you in on even more lore within the rather complex universe.
I didn't expect to like the Hello Neighbor universe as much as my older kids did, but it's actually really interesting. And a surprisingly workable amount of comedy horror that I've never seen pulled off so well. It makes for a great book for any child or adult who is a fan of the game. We were lucky enough to get a look at the Secret Neighbor game and just FYI, it's a great deal of fun. If you or your kids aren't already into this franchise, you probably will be soon. And you're going to need a book to go with it.
My only complaint is that the second book won't be out until after Christmas so it can't be my son's wrapped gift, but he'll be getting a copy of it as an everyday surprise. Can you give kids books as presents on New Year's Eve? Is that a thing? It needs to be a thing. Beats the crap out of resolutions.
At any rate, don't let the fact that these and other titles started out as games first stop you from reading them. We could argue about how games are artforms all day, but the truth is that these games are successful in part because of the amazing stories behind them. Don't miss out on that.