Google operates GSX. They have cute little vans with "Google Shopping Express" painted on them which drive around and deliver the products.

It's whichever local stores agree to partner with Google. As far as I know, the process works like this:

1. Google partners with a store. They place some subset of the inventory of that store online.

2. As a user, you browse that inventory and buy.

3. The store receives the order, places the item into a storage bin in the back and waits for a GSX van to come pick it up.

4. Every so often, a GSX van will swing by, pick up the goods, then deliver.

Yes, you can get books. As far as I know, the only real product you can't get is fresh produce. I think produce has far too low of a volume-profit ratio (and of course, produce spoils if left sitting inside of a hot van for hours on end).