Well, it does get a bit murky. A book is a physical good. An ebook is not. But if I print an ebook out, it is now a physical good.

TL;DR Things are complicated. We keep trying to find physical world analogies for the digital space, and that just isn't going to work well.

Books aren't really physical goods, economically for all intents and purposes they are very similar to digital goods as the cost of reproduction is near zero which is why books like digital goods fall under copyright law.

And what happens when physical goods become similar to digital goods and can be replicated at zero marginal cost?

It won't happen tomorrow, or a year from now, but it'll happen in our lifetime.

I'm betting on UBI / Star Trek, others see violent revolution / Fallout 4.

Essentially, humanity is freed from 'work'

An ebook is still a physical good in a sense. The data (or some portion of it) rests on a medium in your possesion (even if only temporarily). As we all know, there are significant costs involved in creating, advertising, distributing, maintaining said digital good. The costs and subsidies (e.g. open source software, government research, corsortiums) are just distributed in a vastly different way, so much so that to some it may seem to be a free lunch (despite being far from it).

The cost to create and maintain a public good are not relevantly to its "publicness". Only the cost to access it is.

Clean air and national securty also cost money to maintain, but have no user fees.

Digital objects that are torrentable are public goods. This includes media files and local machine software but not web services.

You could torrent a web service if the app was dockerized, the docker containers were part of the torrent file, and the underlying datastore (files or a DB) were also included.