Today, book 30 is on offer. This is showing as already in my library, so it might be a repeat. The Happy Hollisters and the Mystery of the Golden Witch: (Volume 30)
https://smile.amazon.com/gp/product/B07QX8YQ7G
ETA: Forgot to check yesterday, so not sure exactly when it changed, but this is no longer showing free as of 11/6