How far along in the game are you? Are there still recipes to find? There's a counter in the book "100 recipes left"... Not sure if it says anything or just goes away if you've got them all.
If you bought/enabled the complete craftbook iAP you'll stop getting notes too.
New world-gen items won't appear in worlds that were created in previous updates, so you won't find fossils in old worlds. Not sure about the pocket santa.