The economically viable part was beyond question a decade ago.
Nowadays tech has moved on a lot. Not just the programming and programmatic capabilities. We have reliable and accurate GPS receivers for the required navigation. More efficient electric motors, and vastly more efficient batteries (more power, less weight) making these things now capable of actually lifting a significant payload and transporting it over a significant distance (some 16 km or 10 miles iirc - still not far but such a range can easily cover a single city).
Just look at military drones, they also really took off only last decade. Police drones are from the past few years.
Whether they are a cheaper to deliver book than a delivery man on a delivery truck, I doubt. At least for now. Tech moves on though, and these drones will get cheaper.
Delivering a pizza is often more expensive than delivering a book, partly as it is so time sensitive, maybe that is where they will first see real deployment.