I'm not sure that's true. Books can create a DRY problem. When a project moves fast enough, any books written about it have a short window during which they're relevant. After which time they can do as much harm as good, between the info being out of date, or wrong, causing confusion with new developers, diverting them from the official docs, etc.

Flask is pretty small and simple, so books are less necessary than they would be for other types of projects. As long as its own documentation remains good that's going to be the best option.

This is early release, but still fantastic: http://shop.oreilly.com/product/0636920031116.do